This study investigates whether second language (L2) phonological learning can be characterized as a gradual and systematically patterned replacement of nonnative segments by native segments in learners' speech, conforming to a two-stage implicational scale. We adopt a dynamic approach to language variation based on Gatbonton's (1975, 1978) gradual diffusion framework. Participants were 40 Quebec Francophones of different English proficiency levels who produced 80 tokens of English in eight phonetic contexts. In Analysis 1, production accuracy data are subjected to implicational scaling, with phonetic contexts ordered solely by a linguistic criterion--sonority hierarchy. In Analysis 2, the production accuracy data are similarly analyzed but with phonetic context ordering determined by psycholinguistic (processing) criteria--cross-language perceptual similarity and corpus-based estimates of lexical frequency. Results support and extend Gatbonton's framework, which indicates that L2 phonological learning progresses gradually, conforming to an implicational scale, and that perceived cross-language similarity and lexical frequency determine its course.