Cone photoreceptors in the zebrafish retina are arranged in a crystalline lattice, with each spectral subtype at a specific position in the array: rod photoreceptors are inserted around the cones. Patterning events and developmental mechanisms that lead to the formation of the cone mosaic are not known. To begin investigating this issue, we examined the initial stages of opsin expression in zebrafish embryos by in situ hybridization with goldfish opsin cRNA probes to determine how and when the cone mosaic pattern arises. We found both differences and similarities in the spatiotemporal patterns of rod and cone development, which suggest the following: (1) Expression of opsin message (including rod opsin, blue and red cone opsins) was initiated at 50–52 hours postfertilization by a few photoreceptors which were consistently found in a ventral patch of retina located nasal to the choroid fissure. (2) The cone mosaic pattern was generated by a crystallization-like process initiated in the precocial ventral patch and secondarily in nasal retina, which then swept like a wave into dorsotemporal retina. (3) The pattern of differentiation of rods in the ventronasal patch differed substantially from that in the remainder of the retina, suggesting that these precocial rods might differ from typical rods. (4) Developmental maturation of rods in zebrafish, as reflected by expression of opsin, may be accelerated compared to cones, which are thought to become postmitotic before rods. These data are consistent with a model in which lateral inductive interactions among differentiating photoreceptors lead to patterning of the array. © 1995 Wiley-Liss, Inc.