Most empirical research in L2 vowel perception focuses on the development of groups of learners. However, recent studies indicate that individual learners' developmental paths in L2 vowel perception may not be uniform (e.g., Escudero, 2001; Escudero and Boersma, 2004; Morrison, 2009). The aim of the present study is to add to this line of research by investigating (1) whether individual English learners of German follow different paths in their perceptual development of six rounded German vowels, and (2) whether the observed patterns are explicable on the basis of Escudero's (2005) Second-Language Linguistic Perception (L2LP) model. A cross-language perceptual assimilation experiment revealed that learners' assimilation of L2 sounds to native categories is indeed highly diverse, yet systematic. Importantly, these cross-language mapping patterns largely predict the learners' further development in L2 vowel perception, as assessed in a forced-choice identification task. Implications for explanatory frameworks in second-language speech research are discussed.