Children with specific language impairment (SLI) are known to display persistent difficulties with inflectional morphology--in particular, the overuse of unmarked grammatical forms (i.e., zero-marking). Yet, several recent studies have shown that English-speaking children with SLI, like their normal language peers (NL), demonstrate a considerable degree of productive language abilities (e.g., Bishop, 1994; Loeb & Leonard, 1991; Oetting & Horohov, 1997). In this study, we explore productivity in the English past tense in school-age children with SLI (N= 31) and NL (N = 31) who were equivalent as a group in chronological and mental age. Although children in both groups produced a range of error types, the children with SLI produced significantly more errors, with a greater proportion resulting from zero-marking (e.g., go) than suffixation (e.g., goed). Item analyses indicated that suffixations and zero-markings were predicted by item frequency, phonological features of stems, and similarity relationships across items (i.e., neighborhood structure) in both groups, yet children with SLI were more sensitive to item phonology than their NL peers. Results are interpreted in light of the predictions of dual- versus single-mechanism models of morphological productivity. Implications for accounts of SLI are discussed.