<p>During Drosophila oogenesis six to ten follicle cells, the border cells, undergo a dramatic and stereotypic migration through the developing egg chamber. We identified four independent P element insertion mutations that specifically blocked border cell migration. They defined a single, novel locus that was named slow border cells (slbo), because hypomorphic alleles caused delayed onset of the migration. Laser ablation of the border cells, or failure of their migration, caused improper morphogenesis of the micropyle, the egg-shell structure through which the sperm enters at fertilization. The slbo locus was found to encode a product homologous to the CCAAT/enhancer-binding protein (C/EBP), a basic region-leucine zipper transcription factor. Drosophila C/EBP may be required for the expression of gene products mediating border cell migration.</p>