There was a spontaneous outbreak of mycobacteriosis in fancy veiltail guppies, Lebistes reticulatus, raised on an ornamental fish farm in Venezuela. The clinical signs included listlessness, emaciation, spinal curvature, sunken eyes and loss of colour. Numerous acid-fast bacteria, identified as Mycobacterium species, were detected in smears from the kidneys, liver, mesentery and spleen of the fish, from fresh faecal material, and from the unborn embryos of infected gravid females. The bacteria were eradicated by the addition of kanamycin sulphate to the water at a concentration of 50 ppm, the dose being repeated on four occasions with 48 hours between each dose. Fifteen days after the treatment, none of the clinical signs described were detected in any of the treated fish. The offspring bom to treated females were healthy and normal, and did not harbour acid-fast bacteria.