Physiologically identified mitral cells in the carp olfactory bulb were stained by intracellular injection of horseradish peroxidase in order to study the morphology in detail. The somata were fusiform, elongated, oval, triangular, or irregular. The mean diameters of the somata were 30 microns X 14 microns. Two to five thick dendrites arose from the somata and frequently gave off branches to form glomerular tufts. The dendrites extended less than 400 microns; the dendritic field of single mitral cells in the medial or lateral part of the olfactory bulb was confined within the respective part of the bulb. The axons arose from either the somata or the dendrites and had a conical initial portion, usually with a smooth contour. Some cells had poorly developed intrabulbar axon collaterals. No difference between the mitral cells in the medial part of the olfactory bulb and those in the lateral part was found in the soma diameter, the dendritic diameter at the base, or the number of first-order dendrites. However, there was a difference in the site of the origin of the axon between them: most of the axons of the mitral cells in the medial part arose from the dendrites, while most of the axons of the mitral cells in the lateral part arose from the somata. The morphology of physiologically identified mitral cells is basically consistent with that reported in the Golgi studies of teleosts. The limited dendritic fields of mitral cells may underlie the previously reported functional separation of the olfactory bulb into medial and lateral parts. The results also indicate that the two parts of the teleost olfactory bulb are differentiated not only functionally but also morphologically.