In the developing eye ofDrosophila, cell interactions appear to be responsible for organising undifferentiated cells into unit eyes, or ommatidia. Extensive mutagenesis has been used to search for mutations affecting the development and differentiation of ommatidia. These mutations have been characterized using sections of adults and immunocytochemistry of imaginal discs. Fourteen loci on the second chromosome are described that affect the spacing of the preclusters, the differentiation of ommatidial cells, orientation of the ommatidia, or architecture of the adult retina, that cause retinal degeneration in larval or pupal eye discs, or that cause homeotic transformation of part of the head.