diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 34e845442d7..a4568a9616c 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -1630,7 +1630,9 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             Box::new(signed_block.clone()),
         ));
         ops.push(StoreOp::PutState(block.state_root, &state));
+        let txn_lock = self.store.hot_db.begin_rw_transaction();
         self.store.do_atomically(ops)?;
+        drop(txn_lock);
 
         // The fork choice write-lock is dropped *after* the on-disk database has been updated.
         // This prevents inconsistency between the two at the expense of concurrency.
diff --git a/beacon_node/beacon_chain/src/block_verification.rs b/beacon_node/beacon_chain/src/block_verification.rs
index 44dc5eb9f34..aaa0425dd16 100644
--- a/beacon_node/beacon_chain/src/block_verification.rs
+++ b/beacon_node/beacon_chain/src/block_verification.rs
@@ -63,7 +63,7 @@ use std::borrow::Cow;
 use std::convert::TryFrom;
 use std::fs;
 use std::io::Write;
-use store::{Error as DBError, HotColdDB, HotStateSummary, StoreOp};
+use store::{Error as DBError, HotColdDB, HotStateSummary, KeyValueStore, StoreOp};
 use tree_hash::TreeHash;
 use types::{
     BeaconBlock, BeaconState, BeaconStateError, ChainSpec, CloneConfig, EthSpec, Hash256,
@@ -704,6 +704,7 @@ impl<'a, T: BeaconChainTypes> FullyVerifiedBlock<'a, T> {
 
                 // Store the state immediately, marking it as temporary, and staging the deletion
                 // of its temporary status as part of the larger atomic operation.
+                let txn_lock = chain.store.hot_db.begin_rw_transaction();
                 let state_already_exists =
                     chain.store.load_hot_state_summary(&state_root)?.is_some();
 
@@ -727,6 +728,7 @@ impl<'a, T: BeaconChainTypes> FullyVerifiedBlock<'a, T> {
                     ]
                 };
                 chain.store.do_atomically(state_batch)?;
+                drop(txn_lock);
 
                 confirmation_db_batch.push(StoreOp::DeleteStateTemporaryFlag(state_root));
 
diff --git a/beacon_node/store/src/leveldb_store.rs b/beacon_node/store/src/leveldb_store.rs
index b99d66adb14..fc0dea487d7 100644
--- a/beacon_node/store/src/leveldb_store.rs
+++ b/beacon_node/store/src/leveldb_store.rs
@@ -7,12 +7,15 @@ use leveldb::database::Database;
 use leveldb::error::Error as LevelDBError;
 use leveldb::iterator::{Iterable, KeyIterator};
 use leveldb::options::{Options, ReadOptions, WriteOptions};
+use parking_lot::{Mutex, MutexGuard};
 use std::marker::PhantomData;
 use std::path::Path;
 
 /// A wrapped leveldb database.
 pub struct LevelDB<E: EthSpec> {
     db: Database<BytesKey>,
+    /// A mutex to synchronise sensitive read-write transactions.
+    transaction_mutex: Mutex<()>,
     _phantom: PhantomData<E>,
 }
 
@@ -24,9 +27,11 @@ impl<E: EthSpec> LevelDB<E> {
         options.create_if_missing = true;
 
         let db = Database::open(path, options)?;
+        let transaction_mutex = Mutex::new(());
 
         Ok(Self {
             db,
+            transaction_mutex,
             _phantom: PhantomData,
         })
     }
@@ -143,6 +148,10 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
         self.db.write(self.write_options(), &leveldb_batch)?;
         Ok(())
     }
+
+    fn begin_rw_transaction(&self) -> MutexGuard<()> {
+        self.transaction_mutex.lock()
+    }
 }
 
 impl<E: EthSpec> ItemStore<E> for LevelDB<E> {}
diff --git a/beacon_node/store/src/lib.rs b/beacon_node/store/src/lib.rs
index 58be69818e3..38411d9a180 100644
--- a/beacon_node/store/src/lib.rs
+++ b/beacon_node/store/src/lib.rs
@@ -35,6 +35,7 @@ pub use self::partial_beacon_state::PartialBeaconState;
 pub use errors::Error;
 pub use impls::beacon_state::StorageContainer as BeaconStateStorageContainer;
 pub use metrics::scrape_for_metrics;
+use parking_lot::MutexGuard;
 pub use types::*;
 
 pub trait KeyValueStore<E: EthSpec>: Sync + Send + Sized + 'static {
@@ -60,6 +61,12 @@ pub trait KeyValueStore<E: EthSpec>: Sync + Send + Sized + 'static {
 
     /// Execute either all of the operations in `batch` or none at all, returning an error.
     fn do_atomically(&self, batch: Vec<KeyValueStoreOp>) -> Result<(), Error>;
+
+    /// Return a mutex guard that can be used to synchronize sensitive transactions.
+    ///
+    /// This doesn't prevent other threads writing to the DB unless they also use
+    /// this method. In future we may implement a safer mandatory locking scheme.
+    fn begin_rw_transaction(&self) -> MutexGuard<()>;
 }
 
 pub fn get_key_for_col(column: &str, key: &[u8]) -> Vec<u8> {
diff --git a/beacon_node/store/src/memory_store.rs b/beacon_node/store/src/memory_store.rs
index 19308c86d20..2df503965d0 100644
--- a/beacon_node/store/src/memory_store.rs
+++ b/beacon_node/store/src/memory_store.rs
@@ -1,5 +1,5 @@
 use super::{Error, ItemStore, KeyValueStore, KeyValueStoreOp};
-use parking_lot::RwLock;
+use parking_lot::{Mutex, MutexGuard, RwLock};
 use std::collections::HashMap;
 use std::marker::PhantomData;
 use types::*;
@@ -9,23 +9,16 @@ type DBHashMap = HashMap<Vec<u8>, Vec<u8>>;
 /// A thread-safe `HashMap` wrapper.
 pub struct MemoryStore<E: EthSpec> {
     db: RwLock<DBHashMap>,
+    transaction_mutex: Mutex<()>,
     _phantom: PhantomData<E>,
 }
 
-impl<E: EthSpec> Clone for MemoryStore<E> {
-    fn clone(&self) -> Self {
-        Self {
-            db: RwLock::new(self.db.read().clone()),
-            _phantom: PhantomData,
-        }
-    }
-}
-
 impl<E: EthSpec> MemoryStore<E> {
     /// Create a new, empty database.
     pub fn open() -> Self {
         Self {
             db: RwLock::new(HashMap::new()),
+            transaction_mutex: Mutex::new(()),
             _phantom: PhantomData,
         }
     }
@@ -87,6 +80,10 @@ impl<E: EthSpec> KeyValueStore<E> for MemoryStore<E> {
         }
         Ok(())
     }
+
+    fn begin_rw_transaction(&self) -> MutexGuard<()> {
+        self.transaction_mutex.lock()
+    }
 }
 
 impl<E: EthSpec> ItemStore<E> for MemoryStore<E> {}