In standard German, the non-occurrence of -s plurals as the first element of a lexical compound (e.g., *Auto-s-berg ‘heap of cars') vs. the regular occurrence of the other plural suffixes (e.g., Kind-er-club ‘children's club', Frau-en-fete 'women's party') has often been quoted as evidence for a dual mechanism model of morphology, which sees irregular forms as stored in the mental lexicon while regular forms are generated by rule (see Marcus et al. 1995). However, in colloquial northern German, where the s-plural is more widely used than in the standard language, it is possible to form productive compounds containing this suffix (e.g., Mädel-s-treff ‘girls’ meeting'). This paper investigates to what extent -s plurals are acceptable within compounds in colloquial northern German, whether they are subject to any linguistic constraints (for example, morphological or phonological), and what implications they might have for current morphological theory.*