This study attempts to provide extra-theoretical constraints on theories of morpho-syntax by investigating verbal alternations in individuals with aphasia and other language impairments. Thirty-four Hebrew-speaking participants with language impairments were tested with a novel test-battery assessing the production of verbal morphology, focusing on alternating verbs, in comparison to non-alternating and pseudo-alternating verbs. The results show that whereas some participants show a general deficit in morphology causing them to substitute morphological patterns regardless of verb type or argument structure, other patients have a selective deficit in alternating verbs, causing pattern (binyan) substitutions only in alternating verbs, but not in non-alternating verbs and pseudo-alternating with the same surface morphology. Alternant substitution errors occur with all alternating verbs, regardless of alternation type, morphological pattern or valency. This behavior specific to alternating verbs implies that alternating verbs are not an epiphenomenon of roots that appear in different syntactic environments; there is a special theoretical component in their derivation, lexical or syntactic, not found in the derivation of non-alternating verbs.