This study examined stimulus-response properties of somatosensory receptors on the head of rainbow trout, Oncorhynchus mykiss, using extracellular recording from single cells in the trigeminal ganglion. Of 121 receptors recorded from 39 fish, 17 were polymodal nociceptors, 22 were mechanothermal nociceptors, 18 were mechanochemical receptors, 33 were fast adapting mechanical receptors and 31 were slowly adapting mechanical receptors. Mechanical thresholds were higher in polymodal nociceptors than in either slowly adapting or fast adapting mechanical receptors, whereas thermal thresholds of mechanothermal nociceptors were higher than those of polymodal nociceptors. Polymodal nociceptors and mechanochemical receptors gave similar responses to topical applications of acid. All receptor types except mechanothermal nociceptors showed an increase in peak firing frequency with increased strength of mechanical stimulation, with evidence of response saturation at higher intensities. Mechanothermal, but not polymodal, nociceptors showed an increase in firing response to increased temperature. None out of 120 receptors tested gave any response to the temperature range +4 degrees C to -7 degrees C, indicating an absence of cold nociceptors. Attempts to evoke sensitization of receptors using chemical or heat stimuli were unsuccessful, with receptors showing either a return to control responses or irreversible damage. Comparisons are made between somatosensory receptors characterized here in a fish and those of higher vertebrates.