This article reports the findings of a longitudinal study in interlanguage phonology examining the production of two structures in a markedness relationship, biliteral and triliteral onsets, the latter being more marked than the former. This study specifically tests the Interlanguage Structure Conformity Hypothesis (ISCH) by measuring the acquisition of the onsets in the markedness relationship against a criterion measure of 80% correct production. The subjects were 10 native Spanish speakers enrolled in intermediate courses in a community college. Data were gathered twice over a period of 10 months. Results of the study fail to provide any counterevidence to the ISCH.