The present study investigated the relationship between perception and production in the acquisition of word-initial /s/ clusters in the interlanguage of Brazilians learning English as a foreign language. In addition to the interface between perception and production, other factors that might influence both mental processes, e.g., faulty perception, L1 interference, or a combination of these factors, were taken into account. The results part ially support studies which propose that perception influences production. There was also support for the power of L1 interference over Markedness and Universal Canonical Syllable Structure. Epenthesis was the strategy of syllable simplification present in all cluster types. Some subjects, though, resorted to a short epenthetic vowel (/I/), thus indicating that they might be developing a separate category for initial /s/ clusters.