This study examines knowledge of a constraint on the form of synthetic noun-noun compounds in a group of 12 children and adolescents with Williams syndrome (WS; age 8-16 years). The constraint blocks regular plurals from appearing inside compounds (e.g., ferrets breeder) while allowing irregular plurals in the same environment (e.g., mice breeder). In an elicited production task, the WS group showed a strong asymmetry in rates of plurals inside compounds, allowing irregulars liberally but almost never allowing regulars. This demonstrates that people with WS are capable of acquiring and applying combinatorial knowledge that requires the suppression of productive morphological rules, contrary to some recent proposals. These results are compared with other studies of compounding, and the implications of these results for theories of the WS grammatical profile are discussed.