Bilinguals show linguistic transfer effects at several processing levels. Focusing on phonology, we investigate the transfer of optional assimilation rules during speech production. Specifically, we examine to what extent bilinguals apply their native assimilation rule and/or fail to apply an L2 assimilation rule in their L2 speech. Both early and advanced late English-French bilinguals read a short French text. Using a speech recognizer with specific pronunciation variants, we found that the late bilinguals showed evidence for transfer of place assimilation, as well as a reduction in the amount of voicing assimilation compared to that of native French controls. The early bilinguals did not differ from the French controls in terms of place assimilation, but their voicing assimilation rate was intermediate between those of the French controls and the late bilinguals.