Morphological variability in bilingual language production is widely attested. Producing inflected words has been found to be less reliable and consistent in bilinguals than in first-language (functionally monolingual) L1 speakers, even for bilingual speakers at advanced proficiency levels. The sources for these differences are not well understood. The current study presents a detailed investigation of morphological generalization processes in bilingual speakers' language production. We examined past participle formation of German using an elicited-production experiment containing nonce verbs with varying degrees of similarity to existing verbs testing a large group of bilingual Turkish/German speakers relative to L1 German speakers. We compared similarity-based lexical extensions with generalizations of morphological rules. The results show that rule-based generalizations are used less often and more variably within the bilingual group than within the L1 group. Our results also show a selective effect of age of acquisition on the bilingual speakers' morphological generalizations.