The primary olfactory projections of channel catfish Ictalurus punctatus have been examined with postmortem tracing by using either 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate or 1,1-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). Following DiI deposition into small areas in different parts of the posterior olfactory bulb, olfactory sensory neurons always were labeled throughout the olfactory epithelium. No obvious topographical mapping exists between the epithelium and olfactory bulb. The different dye placements, however, did result in labeling of different morphologies of receptor cells, depending on the site of injection. Retrogradely labeled neurons in the olfactory epithelium were classified into three types on the basis of their height: tall, intermediate, and short. Tall olfactory sensory neurons had perikarya at the bottom one-fourth of the epithelium, extended slender dendrites to the epithelial surface, and possessed numerous cilia on the apical dendritic tips. These tall olfactory sensory neurons were labeled predominantly following DiI applications to the ventral part of the posterior olfactory bulb. In contrast, the short olfactory sensory neurons had perikarya situated within the superficial half of the epithelium and with short apical dendrites bearing microvilli. These short olfactory sensory neurons projected predominantly to the dorsal, posterior olfactory bulb. Thus, short microvillous receptor cells and tall ciliated receptor cells connect to different parts of the olfactory bulb, although the receptor cells are intermingled within the olfactory epithelium. Because different parts of the olfactory bulb are thought to respond preferentially to different classes of odorants, these results suggest that receptor cell morphology may be related to odorant quality detection. In addition, to compare this study with previous in vivo studies, Fluoro-Gold was injected in vivo into either the olfactory bulb or intraperitoneally. These in vivo studies show that so-called "type II ciliar receptor cells" of the nonsensory epithelium are labeled nonselectively by blood-borne substances, but they are not labeled by postmortem injections of DiI anywhere in the olfactory bulb.