Drawing upon Schmidt’s (1990, 1993, 1995) claim that attention to form is necessary for acquisition, this study examines how NS-initiated negotiation draws attention to and facilitates acquisition effectively. The study compares two types of negotiation indicators (i.e., clarification requests and recasts) in terms of their effects on attention and acquisition of English question forms in connection with NNSs’ proficiency level. Data from an experiment where 31 Korean college NNSs engaged in information-gap tasks with NSs were analyzed. Results revealed that clarification requests were more effective in drawing attention while recasts were more beneficial in acquisition. This is ascribed to different characteristics of the indicator types and shows that the provision of correctly reformulated form is a key condition for enhancing negotiation’s effects. It was also found that clarification requests’ effects were not related to NNSs’ proficiency level while recasts induced more attention with high level NNSs and facilitated acquisition better with low level ones. It is suggested that indicator type and NNSs’ level be considered in utilizing NS-initiated negotiation as an effective acquisition device. This study implies that the workings of NS-initiated negotiation may be constrained by various factors connected with both learners and interaction they are involved in.