Purpose: Overt marking of "BE" in nonmainstream adult dialects of English is influenced by a number of linguistic constraints, including the structure's person, number, tense, contractibility, and grammatical function. In the current study, the authors examined the effects of these constraints on overt marking of "BE" in children as a function of their nonmainstream English dialect and age. Method: The data were language samples from 62 children, ages 4-6 years; 24 children spoke African American English (AAE), and 38 spoke Southern White English (SWE). Analyses included analysis of variance and logistic regression. Results: Rates of overt marking varied by the children's dialect but not their age. Although the person, number, tense, and grammatical function of "BE" influenced the children's rates of marking, the nature and magnitude of the influence differed by the children's dialect. For AAE-speaking children, contractibility also influenced their marking of "BE". Conclusions: Consistent with the adult literature, the current study showed that AAE- and SWE-speaking children marked "BE" in ways that differed from each other and from what has been documented for child speakers of Mainstream American English. These findings show stability in the use of "BE" in AAE and SWE that spans different generations and different dialect communities. (Contains 4 tables and 2 footnotes.)