This monograph establishes a theoretical model for the investigation of 'incomplete L1 acquisition' in the immigrant situation. The study focuses on the linguistic behaviour of elderly speakers who acquired Yiddish simultaneously with English and have not used the language since childhood. The central question is whether ungrammatical forms in the data can be traced to the attrition of forms once acquired or to the incomplete acquisition of those forms. With regard to Yiddish present perfect rules, results favor the latter explanation, indicating long-fossilized, divergent child language.