Phytophthora sojae is the causal agent of Phytophthora root rot of soybean (PRR), which could attack soybean at any growth stage and cause severe yield loss. Studies were conducted in 2001 and 2002 to determine whether new races had occurred and whether the frequencies of previously existing races had changed in Iowa by testing the purified isolates on eight differential cultivars. Four races that have not been reported previously in Iowa, races 20, 28, 35, and 41, were isolated. Eight isolates were found with seven virulence formula that had not been reported. The percentage of isolates that can overcome Rps 1k has increased from 5% in 1994 to 45% in this study. Dominance of race 3 in P. sojae population on soybean plants reported in previous surveys was replaced by that of races 25 and 35. Races 1 and 3 were still the most populous races in soil. The results suggested that the population composition of P. sojae has shifted, compared to a previous survey that was completed in 1994. Studies were conducted to compare fitness parameters of above P. sojae isolates to explain and predict the population shift of P. sojae. Zoospore production and colony growth rates at different temperatures were compared. Race infection aggressiveness was determined by virulence index value (VIV), calculated by formula VIV=1+(ab−1), where a represents disease incidence (%), and b represents latent period (days/hours). With its medium ability of spore production and infection aggressiveness, race 1 might maintain or lose its popularity in the future. Races 25 and 35 were more aggressive overall and might become the dominant races in the Iowa soybean fields. Races 3 and 28 were of the weakest fitness parameters and might further decrease their proportion in the population. Significant differences among races were observed for colony diameter, zoospore production, and infection aggressiveness, and one or all of these components could be used to explain the shift and even predict the shift. Further study is needed to monitor the population shift, complete the knowledge of fitness characters of races of P. sojae, and to predict the population shift based on the information.