This study investigates the progressive voice assimilation rule in word-final inflectional /z/ in L2 English. We analyzed data from the IPCE-IPAC corpus of learner oral productions, by measuring periodicity for all learner realizations of word-final inflectional /z/. We compared three groups, namely 15 L1 French learners, 15 L1 Italian learners, and 10 L1 Spanish learners. Given the different distributions of /s, z/ in the participants’ L1s and based on SLM and PAM, we expected differences in periodicity across groups. More specifically, we hypothesized that L1 French learners and L1 Italian learners would find it easier than L1 Spanish learners to reproduce the outcome of the voice assimilation rule. Our predictions were only partially reflected by the results: L1 French learners (who have /s/ and /z/ in their L1 as phonemes occurring in word-final position) were indeed the most successful in producing the expected patterns of periodicity. However, L1 Spanish learners were found to be better than L1 Italian learners in doing so. We propose that such results are due to the different phonotactic constraints affecting [s] and [z] in the learners’ L1s. Since Italian phonotactics does not allow [z] in word-final position, our L1 Italian participants produced realizations that are virtually aperiodic. These results have potential repercussions on models of L2 phonology acquisition, since they suggest that L1 phonotactic constraints are an important factor accounting for the pronunciation of an L2.