Here we describe the expression pattern of the smallminded (smid) gene during Drosophila development and investigate the phenotype of a null mutant. In situ hybridisation reveals the ubiquitous expression of smid transcript throughout early embryonic stages until the extended germ band stage, after which expression becomes localised to the neurogenic ectoderm and gonad. Post-embryonic expression is restricted to tissues engaged in the developmental programme of the adult fly: the re-enlarged neuroblasts; imaginal disks; histoblast nests; and precursors of adult muscles. The correlation of smid expression with mitotic activity suggests a cell cycle function which is confirmed by the observed phenotype of a smid null mutant characterised by an abnormally small CNS, due to defective mitosis of post-embryonic neuroblasts and their subsequent death by apoptosis.