In order to establish a model for the investigation of 'incomplete L1 acquisition' - the phenomenon whereby a bilingual speaker incompletely acquires the socially non-dominant language - the author offers a description and analysis of the speech of ten elderly speakers of Yiddish who acquired Yiddish as an L1 simultaneously with English but have not used the language since childhood. Of central interest is the question of whether the divergent forms in the data are the result of L1 attrition - the loss of knowledge once possessed - or incomplete L1 acquisition - lack of initial acquisition...