1. Recent progress in the studies on olfaction in fish, with particular emphasis on electrophysiological and behavioral responses to biological odors and related chemicals, is reviewed. 2. One of the most characteristic features in fish olfaction is that it takes place entirely in the aquatic environment. The carrier of stimulant molecules is not air but water; therefore, chemicals that are detected olfactorily by fish need not be volatile, but must be soluble in water. 3. The olfactory organs of fishes are diversely developed. At one extreme they are well developed (macrosmatic) such as in sharks and eels, and at the other they are poorly developed (microsmatic) such as in pike and stickleback. 4. The nasal cavity is lined with the olfactory epithelium, which is raised from the floor of the organ into a series of lamellae to make a rosette. The arrangement, shape and degree of development of the lamallae in the rosette vary considerably from species to species. 5. It is doubtful whether simple relation exists between the surface area of the olfactory epithelium and sensitivity to odors, since the sensory epithelium is not distributed uniformly over the surface of the olfactory lamellae. 6. The olfactory epithelium of fish, like other vertebrates, consists of three cell types: receptor cells, supporting cells and basal cells. 7. The receptor cell, which is a bipolar primary sensory cell, sends a slender cylindrical dendrite toward the surface of the epithelium and is directly connected with the olfactory bulb by its axon. The dendrite terminates in a minute swelling (olfactory knob) which bears a variable number of cilia. 8. The information from the receptor cell is conveyed into the olfactory bulb, the first relay station, where signals are processed and integrated. The dominant feature of the bulb is the synaptic contact between the primary and secondary olfactory neurones in the form of glomerulus. 9. All the available evidence points to a great acuity of the olfactory sense in many fish species both in the capability and discriminating odorous chemicals. However, much discrepancies exist among data obtained by behavioral and electrophysiological techniques mainly because of the lack of systematic investigations. 10. Electrophysiological studies of olfaction have been hampered by the extremely small size of the olfactory neurones. 11. A slow negative monophasic potential is induced in the olfactory epithelium when stimulated with odorous chemicals (electro-olfactogram, EOG).(ABSTRACT TRUNCATED AT 400 WORDS)