diff --git a/src/stcal/ramp_fitting/ols_fit.py b/src/stcal/ramp_fitting/ols_fit.py
index 5baf8c216..763ed4844 100644
--- a/src/stcal/ramp_fitting/ols_fit.py
+++ b/src/stcal/ramp_fitting/ols_fit.py
@@ -5,6 +5,7 @@
 import warnings
 from multiprocessing import cpu_count
 from multiprocessing.pool import Pool
+import sys
 
 import numpy as np
 
@@ -657,14 +658,15 @@ def ols_ramp_fit_single(ramp_data, buffsize, save_opt, readnoise_2d, gain_2d, we
     opt_info : tuple
         The tuple of computed optional results arrays for fitting.
     """
-
-    use_c = False
-    # use_c = True
+    # use_c = False
+    use_c = True
     if use_c:
         print(" ")
         print("=" * 80)
         c_start = time.time()
 
+        ramp_data, gain_2d, readnoise_2d = endianness_handler(ramp_data, gain_2d, readnoise_2d)
+
         if ramp_data.drop_frames1 is None:
             ramp_data.drop_frames1 = 0
         print("    ---------------- Entering C Code ----------------")
@@ -692,6 +694,70 @@ def ols_ramp_fit_single(ramp_data, buffsize, save_opt, readnoise_2d, gain_2d, we
         c_python_time_comparison(c_start, c_end, p_start, p_end)
 
     return image_info, integ_info, opt_info
+
+
+def handle_array_endianness(arr, sys_order):
+    """
+    Determines if the array byte order is the same as the system byte order.  If
+    it is not, then byteswap the array.
+
+    Parameters
+    ----------
+    arr : ndarray
+        The array whose endianness to check against the system endianness.
+
+    sys_order : str
+        The system order ("<" is little endian, while ">" is big endian).
+
+    Return
+    ------
+    arr : ndarray
+        The ndarray in the correct byte order
+    """
+    arr_order = arr.dtype.byteorder
+    if (arr_order == ">" and sys_order == "<") or (arr_order == "<" and sys_order == ">"):
+        arr.newbyteorder().byteswap(inplace=True)
+    return arr
+
+
+def endianness_handler(ramp_data, gain_2d, readnoise_2d):
+    """
+    Check all arrays for endianness against the system endianness, so when used by the C
+    extension, the endianness is correct.
+
+    Parameters
+    ----------
+    ramp_data : RampData
+        Carries ndarrays needing checked and possibly byte swapped.
+
+    gain_2d : ndarray
+        An ndarray needing checked and possibly byte swapped.
+
+    readnoise_2d : ndarray
+        An ndarray needing checked and possibly byte swapped.
+
+    Return
+    ------
+    ramp_data : RampData
+        Carries ndarrays checked and possibly byte swapped.
+
+    gain_2d : ndarray
+        An ndarray checked and possibly byte swapped.
+
+    readnoise_2d : ndarray
+        An ndarray checked and possibly byte swapped.
+    """
+    sys_order = "<" if sys.byteorder=="little" else ">"
+
+    gain_2d = handle_array_endianness(gain_2d, sys_order)
+    readnoise_2d = handle_array_endianness(readnoise_2d, sys_order)
+
+    ramp_data.data = handle_array_endianness(ramp_data.data, sys_order)
+    ramp_data.err = handle_array_endianness(ramp_data.err, sys_order)
+    ramp_data.groupdq = handle_array_endianness(ramp_data.groupdq, sys_order)
+    ramp_data.pixeldq = handle_array_endianness(ramp_data.pixeldq, sys_order)
+
+    return ramp_data, gain_2d, readnoise_2d
     
 
 
diff --git a/src/stcal/ramp_fitting/src/slope_fitter.c b/src/stcal/ramp_fitting/src/slope_fitter.c
index a6d69f2aa..bb0ccf72d 100644
--- a/src/stcal/ramp_fitting/src/slope_fitter.c
+++ b/src/stcal/ramp_fitting/src/slope_fitter.c
@@ -1785,6 +1785,7 @@ get_ramp_data_endianness(
     rd->get_zframe = get_float3;
 
 # if 0
+    /* Byte swapping now handled in python code. */
     rd->get_data = (PyArray_ISBYTESWAPPED(rd->data)) ? get_float4_swp : get_float4;
     rd->get_err = (PyArray_ISBYTESWAPPED(rd->err)) ? get_float4_swp : get_float4;