Ductin is a putative connexon-forming protein in gap junctions of arthropods. To analyze the role of gap-junction mediated cell-cell communication during Drosophila embryogenesis, we used two different polyclonal anti-ductin sera. One antiserum was directed against ductin isolated from gap junctions of the lobster Nephrops whilst the other was raised against a nonapeptide at the N-terminus of ductin from Drosophila. Both antisera were found to inhibit, when microinjected into Drosophila ovarian follicles, the intercellular exchange of fluorescent tracer molecules between oocyte and follicle epithelium. This result indicates that Drosophila ductin plays a decisive role in gap-junctional communication and confirms the cytoplasmic location of the ductin N-terminus in gap junctions. On immunofluorescence preparations and immunoblots, the anti-ductin sera specifically recognized ovarian as well as embryonic antigens. Following microinjections of the antisera into embryos prior to gastrulation, significantly reduced rates of hatching larvae were obtained. Moreover, microinjections into the mid-ventral region of the embryos resulted in specific ventral defects that depended on the concentration of the ductin antibodies. In particular, larvae with ventral holes in their cuticles occurred with high frequency. During gastrulation, antiserum-injected embryos often developed defects in the middle region of their ventral furrow. Here, mesodermal cells failed to invaginate correctly and, thus, no cuticle was formed. We conclude that, during Drosophila embryogenesis, gap-junctional communication is required for epithelial integrity and morphogenetic events.