Stages in development with rapid transitions between mitosis and morphogenesis may require specific mechanisms to coordinate cell shape change. Here we describe a novel mitotic inhibitor that acts during Drosophila gastrulation to counteract String/Cdc25, specifically in the cells that invaginate to form the mesoderm. We have identified two genes, frühstart and tribbles, that are required for this ventral inhibition. tribbles encodes a kinase-related protein whose RNA, however, is also present outside of the ventral region. Effective inhibition of mitosis in the cells of the ventral furrow depends on the transcription factor Snail that triggers the ventral cell shape changes. When overexpressed in a microinjection assay, Tribbles directly inhibits mitosis. We propose that Frühstart and Tribbles form a link between the morphogenetic movements and mitotic control.