Thyroid hormone receptor beta2 (TRbeta2) controls the patterning of cone opsin photopigments that mediate colour vision. We raised an antiserum against TRbeta2 to study cone photoreceptor development by western blot and immunostaining analyses. TRbeta2-positive cells first appeared between embryonic day 10 (E10) and E12. Numbers increased until near birth, correlating with generation of the cone population. At birth, signals decreased until postnatal day 10, then declined to very low levels in adulthood. TRbeta2-positive cells were initially dispersed but became aligned at the edge of the outer neuroblastic layer by E15. Postnatally, these cells migrated inwardly until postnatal day 10, then outwardly to the edge of the outer nuclear layer, the location of mature cones. TRbeta2 represents a functionally unique marker for cone development.