A fundamental question in the study of second language (L2) fluency is the extent to which temporal characteristics of speakers' first language (L1) productions predict the same characteristics in the L2. A close relationship between a speaker's L1 and L2 temporal characteristics would suggest that fluency is governed by an underlying trait. This longitudinal investigation compared L1 and L2 English fluency at three times over 2 years in Russian- and Ukrainian- (which we will refer to here as Slavic) and Mandarin-speaking adult immigrants to Canada. Fluency ratings of narratives by trained judges indicated a relationship between the L1 and the L2 in the initial stages of L2 exposure, although this relationship was found to be stronger in the Slavic than in the Mandarin learners. Pauses per second, speech rate, and pruned syllables per second were all related to the listeners' judgments in both languages, although vowel durations were not. Between-group differences may reflect differential exposure to spoken English and a closer relationship between Slavic languages and English than between Mandarin and English. Suggestions for pedagogical interventions and further research are also proposed.