The use of the mutual exclusivity constraint was examined in three-year-old and six-year-old children who were either monolingual in English (N=32) or bilingual in English/Urdu (N=32) or in English/Greek (N=32). Three tests of the constraint were used: disambiguation, rejection, and restriction. On the disambiguation test, the mutual exclusivity bias was significantly more evident in five- and six-year-old monolingual children than in their same-age bilingual peers. Monolingual children were also more likely than bilingual children to reject a new name for a familiar object. However, using a restriction test, neither monolingual nor bilingual children readily accepted and restricted typical names for hybrid objects. Developmental differences were also found, as older (five- to six-year-old) monolingual children's responses on the tests were generally more consistent with the constraint than younger (three- to four-year-old) children's responses. Nevertheless, bilingual children did use the constraint, but not to the extent of monolingual children.