The gene crumbs (crb) of Drosophila encodes a transmembrane protein with 30 EGF-like and four laminin A G-domain-like repeats in its extracellular domain. Loss-of-function mutations lead to severe disorganization and degeneration of ectodermally derived embryonic epithelia. In embryos homozygous for crb8F105, an amorphic allele, the CRUMBS protein is diffusely distributed in the cytoplasm instead of being apically localized as in wild-type; this mislocation occurs before any morphologically detectable cellular phenotype becomes manifest, suggesting that apical targeting of proteins is affected in crb mutant embryos. This has been confirmed by using an antibody directed against YELLOW, another apically expressed protein. A single base exchange in crb8F105 leads to the introduction of a premature stop codon, thus eliminating the C-terminal part of the cytoplasmic domain. A possible role for crb in controlling apical-basal polarity is discussed.