Mechanisms generating variation inPhytophthoraare assessed. Patterns of inheritance are typically those of a diploid organism. The increasing availability of single gene markers is allowing the breeding system in heterothallic species to be defined. The genetics of virulence and fungicide resistance is summarized; somatic recombination and variation in chromosome number is discussed. Rapid advances in basic genetics, population genetics and evolution are expected through the application of the new techniques for manipulating DNAin vitro.The complexity of the problem is not underestimated.