This study presents and analyzes the comprehension of relative clauses in child and adult speakers of Russian, comparing monolingual controls with Russian heritage speakers (HSs) who are English-dominant. Monolingual and bilingual children demonstrate full adultlike mastery of relative clauses. Adult HSs, however, are significantly different from the monolingual adult controls and from the child HS group. This divergent performance indicates that the adult heritage grammar is not a product of the fossilization of child language. Instead, it suggests that forms existing in the baseline undergo gradual attrition over the life span of a HS. This result is consistent with observations on narrative structure in child and adult HSs (Polinsky, 2008b). Evidence from word order facts suggests that relative clause reanalysis in adult HSs cannot be attributed to transfer from English.