Exposure to statistical patterns of language use affects language production and comprehension. In this longitudinal study of English language learner (ELL) university students, we examined the interplay between language experience and language statistics as a window into the formation and stability of morphological representations in memory. We hypothesized that within-participant change in sensitivity to distributional properties of complex words on written production would reflect changes in morphological knowledge. At two timepoints, separated by 8 months of language exposure, a sample of ELLs (
n= 196) completed a written suffix completion task. The largest gains in production accuracy were observed for derived words ending in less productive suffixes. In addition, across both timepoints we found a consistent effect of derivational family entropy, such that derived words belonging to morphological families with equally dominant members were less accurately produced. Both effects indicate that ELLs exploit distributional cues to morphological structure and shed light on two aspects of morphological knowledge in ELLs. First, knowledge of suffixes becomes more entrenched in memory, independently of knowledge of the full forms of derived words. Second, ELLs draw upon interlexical connections between morphological family members during written word production.