diff --git a/src/sqomega/_low_level_io.py b/src/sqomega/_low_level_io.py
index 609a734..1e2e8dd 100644
--- a/src/sqomega/_low_level_io.py
+++ b/src/sqomega/_low_level_io.py
@@ -139,8 +139,8 @@ def read_n_chars(self, n: int) -> str:
     def read_array(
         self, shape: tuple[int, ...], dtype: np.dtype[_E]
     ) -> npt.NDArray[_E]:
-        # TODO byteorder
         count = int(np.prod(shape))
+        dtype = dtype.newbyteorder(self.byteorder.value)
         if isinstance(self._file, BytesIO):
             flat = np.frombuffer(
                 self._file.getbuffer(), offset=self.position, dtype=dtype, count=count
@@ -189,9 +189,12 @@ def write_chars(self, value: str) -> None:
 
     @_annotate_write_exception("array")
     def write_array(self, array: npt.NDArray[np.float64]) -> None:
-        # TODO byteorder
         # TODO element order
-        array.tofile(self._file)
+        (
+            array.astype(
+                array.dtype.newbyteorder(self.byteorder.value), copy=False
+            ).tofile(self._file)
+        )
 
     @_annotate_write_exception("bytes")
     def write_raw(self, value: bytes | memoryview) -> None:
diff --git a/tests/write_test.py b/tests/write_test.py
index 491e540..275abcf 100644
--- a/tests/write_test.py
+++ b/tests/write_test.py
@@ -4,6 +4,9 @@
 import sys
 from datetime import datetime, timedelta, timezone
 from io import BytesIO
+from typing import Literal
+
+import pytest
 
 from sqomega import Byteorder, Sqw
 
@@ -75,9 +78,12 @@ def test_create_writes_file_header_big_endian() -> None:
     assert buffer.read(len(expected)) == expected
 
 
-def test_create_writes_main_header() -> None:
+@pytest.mark.parametrize('byteorder', ['native', 'little', 'big'])
+def test_create_writes_main_header(
+    byteorder: Literal['native', 'little', 'big'],
+) -> None:
     buffer = BytesIO()
-    builder = Sqw.build(buffer, title='my title', byteorder="little")
+    builder = Sqw.build(buffer, title='my title', byteorder=byteorder)
     with builder.create():
         pass
     buffer.seek(0)
@@ -90,6 +96,3 @@ def test_create_writes_main_header() -> None:
     assert (main_header.creation_date - datetime.now(tz=timezone.utc)) < timedelta(
         seconds=1
     )
-
-
-# TODO test byteorder of arrays (e.g., in detpar)