The longitudinal study reported in the present paper examines L1 vocabulary decline by a child whose native language input effectively ceased after her immersion into the L2 environment. The subject of the study was a Russian girl adopted by an American family, brought to the USA, and completely isolated from any contact with the Russian-speaking environment. The analysis of the data was based on the results of picture naming tasks and reaction time measurements. Three groups of words showed high vulnerability to forgetting: high frequency words, cognates, and semantically convergent pairs (pairs of words lexically distinguished in L2 and non-distinguished in L1). Fast forgetting of these lexical items in L1 was related to the acquisition of their equivalents in L2. The comparison of noun versus verb retention/acquisition suggested that there might be a delay in L1 verb forgetting / L2 verb acquisition at the early stage of an extensive exposure to the second language.