AbstractThe paper takes plural formation as an example of morphological assimilation of loanwords into the German inflectional system and shows that the assimilation process proceeds in two steps. German speakers first create intermediate plural forms with the non syllabic suffix -s, later the definite forms with a schwa suffix. Both kinds of plural forms are analysed in the framework of Optimality Theory. The forms created during the two stages are shown to follow from different hierarchies of wellformedness constraints and correspondence constraints respectively. Correspondence constraints are highly ranked for unassimilated borrowings as they are for peripheral word classes of proper names and onomatopoeia, by contrast, prosodic and phonological, esp. phonotactic wellformedness constraints are highly ranked for assimilated borrowings as they are for common nouns in the native lexicon. The shape of both kinds of plural forms will be shown to be functionally motivated. Whereas the schwa plural formation is rooted in the pressure for plural forms to be easy to articulate, the -s plural is rooted in the need for preservation of the sound shape of the stems: The syllabic schwa suffixes provide trochaic plural forms, words of optimal length with optimal syllable structure, the non syllabic -s provides structure preserving forms where the base is easy to recognize.