Children pass through a stage in development when they produce utterances that contain auxiliary BE (he's playing) and utterances where auxiliary BE is omitted (he playing). One explanation that has been put forward to explain this phenomenon is the presence of questions in the input that model S-V word order (Theakston, Lieven & Tomasello, 2003). The current paper reports two studies that investigate the role of the input in children's use and non-use of auxiliary BE in declaratives. In Study 1, 96 children aged from 2;5 to 2;10 were exposed to known and novel verbs modelled in questions only or declaratives only. In Study 2, naturalistic data from a dense database from a single child between the ages of 2;8 to 3;2 were examined to investigate the influence of (1) declaratives and questions in the input in prior discourse, and (2) the child's immediately previous use of declaratives where auxiliary BE was produced or omitted, on his subsequent use or non-use of auxiliary BE. The results show that in both the experimental and naturalistic contexts, the presence of questions in the input resulted in lower levels of auxiliary provision in the children's speech than in utterances following declaratives in the input. In addition, the children's prior use or non-use of auxiliary BE influenced subsequent use. The findings are discussed in the context of usage-based theories of language acquisition and the role of the language children hear in their developing linguistic representations. © 2008 Cambridge University Press.