diff --git a/tests/test_xwakes_headtail_table_read.py b/tests/test_xwakes_headtail_table_read.py
index faf16821..48d8c239 100644
--- a/tests/test_xwakes_headtail_table_read.py
+++ b/tests/test_xwakes_headtail_table_read.py
@@ -34,7 +34,7 @@ def test_headail_table_read():
     for i_component, component in enumerate(wake_file_columns):
         if component != 'time':
             if component == 'longitudinal':
-                conversion_factor = 1E12
+                conversion_factor = -1E12
             else:
                 conversion_factor = 1E15
 
diff --git a/xwakes/read_headtail_table.py b/xwakes/read_headtail_table.py
index 31e4bff8..5ae4aeb7 100644
--- a/xwakes/read_headtail_table.py
+++ b/xwakes/read_headtail_table.py
@@ -36,7 +36,7 @@ def read_headtail_file(wake_file, wake_file_columns):
         if component != 'time':
             assert component in valid_wake_components
             if component == 'longitudinal':
-                conversion_factor = 1E12
+                conversion_factor = -1E12
             else:
                 conversion_factor = 1E15
 
@@ -45,4 +45,4 @@ def read_headtail_file(wake_file, wake_file_columns):
 
     df = pd.DataFrame(data=dict_components)
 
-    return df
\ No newline at end of file
+    return df