diff --git a/cij/util/fill.py b/cij/util/fill.py
index 01faf83..9ec8342 100644
--- a/cij/util/fill.py
+++ b/cij/util/fill.py
@@ -8,6 +8,8 @@
 import numpy
 from sympy.parsing.sympy_parser import parse_expr
 
+from cij.data import get_data_fname
+
 
 def fill_cij(
     elast: pandas.DataFrame,
@@ -49,8 +51,17 @@ def fill_cij(
     a = numpy.array(a)
     b = numpy.array(b)
 
+
     # constraints
 
+    # ... try to find constraints file
+
+    if not Path(constraints).exists():
+        constraints = Path("constraints") / constraints
+        constraints = get_data_fname(str(constraints))
+
+    # ... apply constraints
+
     eqns = []
 
     with open(constraints) as fp: