Two types of on-center bipolar cells, rod- and cone-dominant bipolars have been identified in the dark-adapted retina of the carp by means of intracellular recordings and Lucifer-yellow dye injection. They differ physiologically and morphologically in the following respects: 1) responses of rod-dominant cells to bright lights are characterized by a transient depolarization followed by a smaller sustained depolarization, while those of cone-dominant cells are approximately rectangular; 2) the cone-dominant cells are about 1.5 log units less sensitive to light than the rod-dominant cells; 3) the latency of the response is shorter in the cone-dominant cells than in the rod-dominant cells; 4) the mean diameters of the cone-dominant cell receptive field (0.7 mm) and dendritic field (90 micrometers) are larger than those of the rod-dominant cell receptive field (0.5 mm) and dendritic field (56 micrometers); 5) the mean diameter of the cone-dominant cell soma (8 micrometers) is smaller than that of the rod-dominant cell soma (13 micrometers); and 6) the terminations of the cone-dominant cell axons form a ramification (67 micrometers mean diameter) in contrast to a big terminal swelling of the rod-dominant cell axons (37 micrometers mean diameters). At least two ionic mechanisms are responsible for generating the depolarizing response of on-center bipolar cells, one having a reversal at a positive potential and the other at a negative potential. Responses with a negative reversal potential only are obtained from some of cone-dominant cells and responses with a positive reversal potential only are obtained from some other cone-dominant cells and the rod-dominant cells. There are a large number of bipolar cells that respond by both ionic mechanisms, although the ratio between them varied considerably in different cells.