We investigate the acquisition of /s/ - /z/ in L2 English by comparing contexts in which these sounds have a phonemic value vs contexts in which they are determined by a voice assimilation rule. We observe English L2 productions by French L1, Northern Italian L1 and Southern American Spanish L1 learners, on the assumption that the three groups will show different patterns depending on the status of [s] and [z] in their L1s.