Native language (L1) attrition is ubiquitous in modern globalized society, but its cognitive/psycholinguistic mechanisms are poorly understood. We investigated lexico-semantic L1 attrition in L1 Russian immigrants in Israel, who predominantly use their second language (L2), Hebrew, in daily life. We included Russian monolinguals as a control group. We tested two potential causal mechanisms of attrition: L2 interference versus L1 disuse. Participants completed a fill-the-gap task in two conditions: accuracy (producing one exactly matching word) and scope (providing as many synonyms as possible). We expected L2 interference and L1 disuse to lead to the differential reduction of accuracy and scope features, respectively. Lower scores for attriters emerged in the accuracy but not in the scope condition. Moreover, attitude towards L1 influenced attriters' accuracy--but not scope--performance, with higher L1 preference predicting higher accuracy. We provide evidence for lexico-semantic attrition in adult immigrants, pointing to L2 interference as the primary cause of impaired lexical retrieval.