Rods and cones are morphologically and developmentally distinct photoreceptor types with different functions in vision. Cones mediate daylight and color vision and in most mammals express M and S opsin photopigments for sensitivity to medium-long and short light wavelengths, respectively. Rods mediate dim light vision and express rhodopsin photopigment. The transcription factor networks that direct differentiation of each photoreceptor type are incompletely defined. Here, we report that Rorb^−/− mice lacking retinoid-related orphan nuclear receptor β lose rods but overproduce primitive S cones that lack outer segments. The phenotype reflects pronounced plasticity between rod and cone lineages and resembles that described for Nrl^−/− mice lacking neural retina leucine zipper factor. Rorb^−/− mice lack Nrl expression and reexpression of Nrl in Rorb^−/− mice converts cones to rod-like cells. Thus, Rorb directs rod development and does so at least in part by inducing the Nrl-mediated pathway of rod differentiation.