This study presents evidence of ultraviolet (UV) sensitive, ON center ganglion cells in the fish retina. We determined the spectral sensitivity of ON and OFF responses from the optic nerve mass potential in small (18.0 - 28.5 g) and large (59.5-835 g) rainbow trout, with special reference to UV sensitivity. Under a mid+long-wavelength adapting background, the ON response of small fish revealed the presence of a UV cone mechanism (lambda max 390 nm) which was absent in large specimens. Under similar background conditions, the OFF response of both small and large fish showed one sensitivity peak, dominated by inputs from an M-cone mechanism. An almost complete absence of the accessory corner cones from the retinal mosaic was correlated with the loss of UV sensitivity.