In parasitic-gap constructions an illicit gap inside a syntactic island becomes acceptable in combination with an additional licit gap, a result that has interesting implications for theories of grammar. Such constructions hold even greater interest for the question of the relation between grammatical knowledge and real-time language processing. This article presents results from two experiments on parasitic-gap constructions in English in which the parasitic gap appears inside a subject island, before the licensing gap. An offline study confirms that parasitic gaps are acceptable when they occur inside theinfinitival complement of a subject NP, but not when they occur inside a finite relative clause. An on-line self-paced reading study using a plausibility manipulation technique shows that incremental positing of gaps inside islands occurs in just those environments where parasitic gaps are acceptable. The fact that parasitic gaps are constructed incrementally in language processing presents a challenge for attempts to explain subject islands as epiphenomena of constraints on language processing and also helps to resolve apparent conflicts in previous studies of the role of island constraints in parsing.