An investigation was carried out to determine whether transmission of passive immunity from the mother to the fry occurred in the guppy, Poecilia reticulata. Female guppies were immunized by an intraperitoneal injection of formalin-killed Aeromonas hydrophila bacterin. Agglutinating antibody titers and the precipitating antibody in the body fluid of female guppies and their fry were investigated. The protection against the A. hydrophila pathogen by maternal immunity was also tested. Four weeks after immunization, 14 fry from immunized and nonimmunized females were challenged with 5-min exposure to a virulent A. hydrophila at dosages of 1 x 10(7) cfu/ml. Agglutinating antibody titers in the fry increased, ranging from 1/16 to 1/32. Body fluid of the newborn fry showed a precipitin line against sonicated A. hydrophila. In the challenge, the cumulative mortality of fry from the control fish (43%) was higher than that of fry from immunized fish (14%). The relative percentage protection of fry from immunized females was 67%. The results suggest that the antibody was transmitted from the mother to the newborn fry and the transfer of maternal immunity provides resistance to A. hydrophila in the guppy.