The acquisition of WF-rules by children learning their mother-tongue has hardly been studied up to now for English or Dutch. Yet, it seemed to me that the acquisition of WF proceeds much more slowly than the acquisition of phonology, syntax and flexional morphology. To test this hypothesis a WF-test of 211 items was submitted to a sample of 109 thirteen-year-old children of different social categories and to a control group of 20 adults. An adapted version has been submitted later on to a sample of 80 sixteen-year-old adolescents. Both versions try to measure the knowledge of WF- rules, and not the knowledge of certain morpho-logically complex words. It appears from the test that the children master only half of the WF of Dutch: only 47 % of the items have been correctly formed, understood or judged. Even using a less rigid criterion - not mastery, but WF-ability in general - they don't do much better (55 %). The lexical-morphological ability of the adolescents reaches 69 %; i.e. one fourth higher than that of the children. The proportion of normative answers rises from 39 % of all answers given by children over 54 % for the adolescents to 73 % for the adults. The author's conclusion that children - and to a lesser extent, adolescents -do not master Dutch WF-rules, is illustrated with the discussion of four test items. The author stresses the desirability of teaching WF-rules at the right age and of correctly distinguishing between knowledge of words-the norm- and knowledge of the rules to form and understand complex words, i.e. the language-system.