Purpose: In this study, the authors aimed to explore the relationship between lexical and phonological knowledge in children with primary language impairment (PLI) through the application of a developmental methodology. Specifically, they tested whether there is evidence for an impairment in the process of phonological abstraction in this group of children over and above that which would be predicted by their vocabulary growth. Method: The authors measured developmental change in the abilities of typically developing (TD) children ("n" = 38, chronological age [CA] = 3;0-5;6 [years; months]) and those with PLI ("n" = 13, CA = 3;0-6;6 and 4;6-8;0) to repeat nonwords with high and low phonotactic probability (PP) to uncover group differences in the process of abstraction of phonological representations. Results: Cross-sectional developmental trajectories of change in nonword repetition abilities were compared across CA and vocabulary growth. Unlike TD children, the children with PLI did not evince a narrowing gap in the influence of PP across development, and they reached a plateau in their development. Conclusion: These results suggest slowed emergence of phonological representations in PLI, with a plateau in the development of lexical-phonological representations. This plateau may represent entrenchment and "fixing" due to a missed critical period and/or atypical word learning biases in PLI.