The gene crumbs (crb) of Drosophila melanogaster provides an essential function for the embryonic development of ectodermally derived epithelia. Complete loss of function alleles of the crb gene are recessive embryonic lethals and lead to a disorganization of the primordia of these epithelia, followed by cell death in some tissues. In crb mutant embryos, different organs are affected to a different extent. Some tissues die almost completely (as the epidermis, the atrium and the pharynx) while others partially survive and conserve their basic epithelial structure (as the tracheal system, the oesophagus, the proventriculus, the salivary glands, the hindgut and the Malpighian tubules). Degeneration is first visible at stage 11 and continues successively throughout development. There is evidence that the loss of epithelial cell polarity may be the cause for the degeneration of these tissues, suggesting that the crb gene product is involved in stabilizing the apico-basal polarity of epithelial cells. As previously shown, the crb protein is specifically expressed on the apical side of embryonic epithelia in a reticular pattern outlining the borders of the cells. Here we demonstrate that the crb protein shows the same subcellular localization in epithelial cells of imaginal discs and in follicle cells, indicating a similar function of crb during the development of embryonic, imaginal and follicle epithelia. Clonal analysis experiments indicate that the genecrb is not cell-autonomous in its expression, suggesting that the gene product may act as a diffusible factor and may serve as a signal in a cell-cell communication process. This signal is thought to be required for the formation and/or maintenance of the cell and tissue structure of the respective epithelia.