Data on body size, age/size at sexual maturity, and adult survival were obtained between 1975 and 1978 for a population of bullfrogs, Rana catesbeiana, at the E. S. George Reserve of the University of Michigan. Both males and females continued to grow when adults; older individuals were often 50% larger than newly matured individuals. No sexual differences in adult growth rate were observed, but data on average and maximum adult body size indicated that females were significantly larger than males during each year of the investigation. The discrepancy between these measures resulted from sex—specific differences in adult age distribution: males matured at earlier ages and/or smaller sizes than females, and large males suffered higher mortality than large females. Such life history differences between the sexes can in turn be related to patterns of mating effort and parental effort. Apparently, selection for mating effort in males to defend territories and parental effort in females to produce large clutches has resulted in sexual maturation at different ages but in similar advantages of large body size. The influence of mating effort and parental effort on sexual dimorphism is discussed for other anuran species as well as birds and mammals. It is suggested that an observed body size dimorphism in any species in which individuals grow appreciably as adults could result from a difference in the age distributions of males and females as well as from differential growth rates.