diff --git a/pyteltools/slf/Serafin.py b/pyteltools/slf/Serafin.py
index 9f818fd..cb0b824 100755
--- a/pyteltools/slf/Serafin.py
+++ b/pyteltools/slf/Serafin.py
@@ -1110,22 +1110,22 @@ def write_header(self, header):
         # IKLE
         nb_ikle_values = header.nb_elements * header.nb_nodes_per_elem
         self.file.write(header.pack_int(4 * nb_ikle_values))
-        self.file.write(np.array(header.ikle, dtype=np.dtype(np.int32).newbyteorder(header.endian)).tobytes())
+        self.file.write(np.array(header.ikle, dtype=np.dtype(np.int32).newbyteorder(header.endian)))
         self.file.write(header.pack_int(4 * nb_ikle_values))
 
         # IPOBO
         self.file.write(header.pack_int(4 * header.nb_nodes))
-        self.file.write(np.array(header.ipobo, dtype=np.dtype(np.int32).newbyteorder(header.endian)).tobytes())
+        self.file.write(np.array(header.ipobo, dtype=np.dtype(np.int32).newbyteorder(header.endian)))
         self.file.write(header.pack_int(4 * header.nb_nodes))
 
         # X coordinates
         self.file.write(header.pack_int(header.float_size * header.nb_nodes))
-        self.file.write(np.array(header.x_stored, dtype=header.np_type).tobytes())
+        self.file.write(np.array(header.x_stored, dtype=header.np_type))
         self.file.write(header.pack_int(header.float_size * header.nb_nodes))
 
         # Y coordinates
         self.file.write(header.pack_int(header.float_size * header.nb_nodes))
-        self.file.write(np.array(header.y_stored, dtype=header.np_type).tobytes())
+        self.file.write(np.array(header.y_stored, dtype=header.np_type))
         self.file.write(header.pack_int(header.float_size * header.nb_nodes))
 
     def write_entire_frame(self, header, time_to_write, values):
@@ -1144,5 +1144,5 @@ def write_entire_frame(self, header, time_to_write, values):
 
         for i in range(header.nb_var):
             self.file.write(header.pack_int(header.float_size * header.nb_nodes))
-            self.file.write(np.array(values[i, :], dtype=header.np_type).tobytes())
+            self.file.write(np.array(values[i, :], dtype=header.np_type))
             self.file.write(header.pack_int(header.float_size * header.nb_nodes))