Dopaminergic and serotonergic cells were visualised immunohistochemically with antibodies against tyrosine hydroxylase and serotonin in retinal wholemounts of eight teleosts from different habitats and with different rod-to-cone ratios. The cell densities were calculated, and the density ratio of dopaminergic cells versus serotonergic cells was compared among these fish species. The density ratio was high (1.9-2.7) in three out of the four species of cichlid fish studied with cone densities roughly equalling rod densities, medium in roach (0.8) where rods dominate cone numbers, and low in deep-sea fish (0.2-0.4) with pure rod retinas. These observations confirm earlier findings on the species-specificity of the ratio of dopaminergic versus serotonergic cells in the retina and further demonstrate a close correlation between the rod-to-cone ratio, and the density ratio of dopaminergic versus serotonergic cells in the inner retina. The possible significance for the processing of photopic and scotopic information in the inner retina is discussed.