Swain's (1985, 1995, 2000) output hypothesis states that language production is facilitative of second language (L2) learning. An important component of the output hypothesis involves "pushing" learners to produce appropriate, accurate, and complex language (Swain, 1993), which may occur when interlocutors provide learners with negative feedback (Gass, 1997, 2003; Long, 1996; Mackey, in press; Pica, 1994; Swain & Lapkin, 1995). When learners modify their previous utterances in response to negative feedback, learning opportunities are created by both the provision of negative feedback and the production of modified output. Consequently, it is difficult to determine how these interactional features--alone or in combination--positively impact L2 development. The current study examines the impact of negative feedback and learners' responses on English as a second language (ESL) question development, which is operationalized as stage advancement in Pienemann and Johnston's developmental sequence for ESL question formation (Pienemann & Johnston, 1987; Pienemann, Johnston, & Brindley, 1988). Thai English as a foreign language (EFL) learners (n = 60) carried out a series of communicative tasks with native English speakers in four conditions that provided different negative feedback and modified output opportunities and also completed four oral production tests over an 8-week period. Analysis of the treatment data identified the amount of modified output involving developmentally advanced question forms produced by the learners, and analysis of the test data revealed whether the learners' stage assignment changed over time. Logistic regression indicated that the only significant predictor of ESL question development was the production of modified output involving developmentally advanced question forms in response to negative feedback. (Contains 1 footnote.)