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Early fry (30 d postfertilizalion) and 7-8-week-old Mozambique tilapias (Tilapia mossambicd) were exposed to X rays in dosages of 50, 100, 200, 300, 400, or 500 roentgens and reared in outdoor culture tanks between May 1981 and October 1988. Fish of either sex that were irradiated as fry grew faster than controls at all test X-ray doses. Among fish irradiated at age 7-8 weeks, males grew significantly faster, but females grew significantly slower, than controls at all test doses. X-ray-induced changes in growth were dose-dependent: growth rates of fry (both sexes) and of juvenile males rose relative to those of controls with increased radiation dose. The growth increase per unit of radiation dose was higher for fry than for older juveniles. The lengthweight regression was steeper for irradiated males than for controls. The average weights of Fj offspring of irradiated fish were greatly reduced as compared with controls, which suggests the transfer of the detrimental effects of X rays from irradiated parents to their offspring.