The emergence of verb phrase ellipsis is investigated in two 2-year-old English-speaking children's speech by studying their answers to yes/no questions over the period of about a year. The investigation is framed using a generative linguistics model, the "PF-deletion model," which assumes that there is syntactic structure in the ellipsis site that is simply not pronounced at the level of Phonological Form (PF). On this model, if a child's use of elliptical verb phrases is delayed, in principle he or she could pronounce the material that would otherwise be elided and produce full sentences instead. Two potential reasons for delay in verb phrase ellipsis are considered: (i) learning of the auxiliary system of English and (ii) the intonation requirements on the auxiliary verb. One child produced sentences with verb phrase ellipsis early in the course of acquisition, although there were gaps in her knowledge of the auxiliary system. The second child did not produce verb phrase ellipsis for several months and, instead, initially produced full sentence answers to yes/no questions a high proportion of the time. This child appeared to refrain from using VP ellipsis until the intonational requirements of its use had been mastered, a finding that is consistent with the PF-deletion model.