There is significant concern that high‐intensity sounds can impact fish physiology and behavior. Important sources of such sounds are commercial and military sonars. A recent publication reports that exposure to low‐frequency active (LFA) sonar might result in hearing loss in some fish species, but did not damage any tissues [Popper et al. (2007). J. Acoust. Soc. Am. 122, 623–635]. We have extended our studies to include U.S. Navy mid‐frequency sonar. Fish were exposed to a 3‐s‐long signal that consisted of a 2‐s 2.8–3.8‐kHz frequency sweep, immediately followed by a 1‐s 3.3‐kHz tone). The stimulus was repeated five times with a 25‐s interval. Fish were subsequently tested for hearing sensitivity and examined both grossly and microscopically for tissue damage. Some temporary hearing loss was found in catfish, species known to hear sounds above 1000 Hz, whereas there was no effect in fish which do not hear above about 1 kHz. There was no gross damage to any tissue, and microscopic examination showed no effect on any tissues. [Work supported by Office of the U.S. Chief of Naval Operations.]