Bioaccumulation of 137Cs was investigated in yellow bullhead catfish (Ameiurus natalis) inhabiting an abandoned reactor reservoir, Pond B, Savannah River Site, Barnwell Co., South Carolina. We collected fish by trap-netting, and determined ages from pectoral spines. Muscle and other tissues were assayed for 137Cs by NaI-scintillation. Muscle 137Cs was unrelated to sex (p = 0.859) or mass of fish (p = 0.224), but was related to age (p = 0.036). Examination of least-squares means suggested that 137Cs in muscle increased up to about age 3, but did not increase with greater age (means for 1, 2, 3, 4, 5, and 6 year olds were 2,630, 3,011, 3,513, 3,417, 3,599, and 3,339 Bq/kg, respectively). A modified Richards model showed equilibrium 137Cs concentration in muscle (3,625 Bq/kg) was acquired in approximately 2.4 years. Growth differed between sexes (p < 0.001), and the time to asymptotic body mass was longer than the time to attain equilibrium 137Cs concentration. Males attained an asymptotic mass of 577 g in approximately 6.3 years; females attained an asymptotic mass of 438 g in approximately 5.9 years. The cumulative 137Cs burden of the population was 4.9 × 106 Bq, representing <0.001% of the 137Cs inventory of the reservoir. Concentration of 137Cs varied among tissues (p < 0.001), with gill and muscle the lowest and highest (634 and 3,712 Bq/kg, respectively). Concentration of 137Cs in ovaries declined with increasing ovary mass (p = 0.001; r2 = 0.60). Until equilibrium is attained in these fish, 137Cs concentration is directly related to increasing age rather than size.