Children's acquisition of the mass-count distinction in English was investigated. In order to determine whether children approach the distinction as a morphosyntactic or a semantic distinction, 88 monolingual children aged 3-9 years were asked to judge the acceptability of 32 sentences containing "much" or "many" with 8 types of nominals. The results suggest that a prolonged development of the mass-count distinction in English begins as early as age 3 and may not be completed until after age 9. By age 6 or 7, children may be aware of the correct forms of mass and count nouns when they occur in quantified constructions. There is some indication that by age 8, children may progress to a more semantic understanding of the distinction, in which nominal forms become more flexible in distribution and the mass-count distinction is transferred to the quantifiers modifying those nouns. (RW)