diff --git a/fields/string.py b/fields/string.py
index 4c34731..091575c 100644
--- a/fields/string.py
+++ b/fields/string.py
@@ -221,6 +221,23 @@ class String(Field):
     def MODULO_97_COMPLEMENT(obj, value):
         return String._MODULO_97(obj, value, True)
     BELGIAN_ENTERPRISE_NUMBER = MODULO_97_COMPLEMENT
+
+    @staticmethod
+    def BELGIAN_NISS(obj, value):
+        '''Returns True if the NISS in p_value is valid.'''
+        if not value: return True
+        # Remove any non-digit from nrn
+        niss = sutils.keepDigits(value)
+        # NISS must be made of 11 numbers
+        if len(niss) != 11: return False
+        # When NRN begins with 0 or 1, it must be prefixed with number "2" for
+        # checking the modulo 97 complement.
+        nissForModulo = niss
+        if niss.startswith('0') or niss.startswith('1'):
+            nissForModulo = '2'+niss
+        # Check modulo 97 complement
+        return String.MODULO_97_COMPLEMENT(None, nissForModulo)
+
     @staticmethod
     def IBAN(obj, value):
         '''Checks that p_value corresponds to a valid IBAN number. IBAN stands
diff --git a/shared/utils.py b/shared/utils.py
index 573c183..f71e5d5 100644
--- a/shared/utils.py
+++ b/shared/utils.py
@@ -227,12 +227,19 @@ def normalizeString(s, usage='fileName'):
         res = s
     return res
 
-# ------------------------------------------------------------------------------
 def normalizeText(s):
     '''Normalizes p_s: remove special chars, lowerizes it, etc, for indexing
        purposes.'''
     return normalizeString(s, usage='extractedText').strip().lower()
 
+def keepDigits(s):
+    '''Returns string p_s whose non-number chars have been removed.'''
+    if s is None: return s
+    res = ''
+    for c in s:
+        if c.isdigit(): res += c
+    return res
+
 # ------------------------------------------------------------------------------
 def formatNumber(n, sep=',', precision=2, tsep=' '):
     '''Returns a string representation of number p_n, which can be a float