The Drosophila eye consists of a reiterative hexagonal array of photoreceptor cell clusters, the ommatidia. During normal morphogenesis, the clusters in the dorsal or ventral halves of the disc rotate 90 degrees in opposite directions, forming mirror images across a dorsoventral equator. In the mutant nemo (nmo), there is an initial turning of approximately 45 degrees, but further rotation is blocked. Genetic mosaic analysis indicates that the nmo gene acts upon each cluster as a whole; normal nmo function in one or more photoreceptor cells appears to be sufficient to induce full rotation. The nmo gene sequence encodes a serine/threonine protein kinase homolog, suggesting that the kinase is required to initiate the second step of rotation. In another mutant, roulette, excessive rotation through varying angles occurs in many ommatidia. This defect is suppressed by nmo, indicating that nmo acts upstream in a rotation-regulating pathway.