In Drosophila, ventral furrow formation and mesoderm differentiation are initiated by two regulatory genes, twist (twi) and snail (sna). Both genes are evolutionarily conserved and have also been implicated in vertebrate gastrulation. Evidence is presented that sna is sufficient to initiate the invagination of the ventral-most embryonic cells in the absence of twi+ gene activity. The invaginated cells fail to express mesoderm regulatory genes, suggesting that ventral furrow formation can be uncoupled from mesoderm differentiation. Despite the previous demonstration that sna functions as a sequence-specific transcriptional repressor, low levels of sna that fail to repress neuroectoderm determinants in the presumptive mesoderm are nonetheless able to promote invagination. Cells that possess an ambiguous developmental identity can initiate the invagination process, providing further evidence that ventral furrow formation need not be linked to mesoderm differentiation.