It is generally considered advantageous for populations to periodically recombine their genes in order to increase both the genetic variability and the rate of response to changing environments. Theoretical and experimental evidence, however, has not clearly substantiated this view and actually has led to some confusion about the role of gene recombination in the origin and maintenance of traits of aggressiveness in pathogen populations. Therefore, field and growth chamber studies were conducted to compare sexual and asexual populations of Puccinia coronata for traits of aggressiveness. In 1979, aecial cultures of Puccinia coronata were collected from the alternate host, Rhamnus cathartica, in a nursery in Minnesota to obtain a sample population that had undergone sexual reproduction. A second population distant from R. cathartica consisted of asexual uredial cultures from southern Texas. Isolates obtained from these cultures were used individually to inoculate susceptible oat (Avena sativa) cultivars in both studies. The growth chamber study indicated that the sexual population exhibited significantly higher levels of aggressiveness than the asexual population for several traits, including spore production and latent period;Gene recombination provided no consistent advantage, however, to the sexual population in the field study. This may be due to the absence of the effects of genetic drift in the two large pathogen populations. The general response to selection for increased levels of aggressiveness in the asexual population was small compared to that of the sexual population on the cultivars Lang and Otee which suggests that the advantage of gene recombination will vary over changing host populations and fluctuating environments.