The structure and dynamics of races of the potato late blight pathogen Phytophthora infestans in several populations collected worldwide during 1966–1993 were investigated using mathematical estimates of phenotypic diversity (Shannon and Rogers indices) and of virulence complexity. As expected, the highest diversity of races was found in central Mexico, whereas European and other American populations had consistently lower numbers of races and were usually dominated by one or a few phenotypes. Rogers indices were generally close to 0.5 when calculated for populations collected over successive years from the same area and for populations from neighbouring regions collected at the same period of time, indicating that some geographical and temporal structuring was the rule. This is consistent with a metapopulation structure for P. infestans and provides evidence for evolutionary patterns driven by founder effects. The mean number of virulence genes per race was highly variable among populations. Unnecessary virulences were frequent in most if not all populations. In all populations investigated, the rarest virulence genes were found in highly complex races, indicating a predominant influence of mutation events on race evolution. Key words: potato, late blight, virulence, diversity, complexity, population structure.