Pathways of genetic transfer outside of the normal sexual cycle were examined in Phytophthora infestans. Transformants expressing genes for resistance to G418 or hygromycin were employed to select hybrids or recombinants on double-drug media after in vitro or in planta mixing of zoospores or mycelia. In experiments using pairs of isolates of the same mating type, zoospores fused to form doubly resistant colonies. These were shown to be heterokaryons containing non-recombinant parental nuclei based on the analysis of DNA markers in the hybrids and their single-nuclear derivatives. Mixtures of vegetative hyphae failed to produce hybrids. The application of the drug selection scheme to mating cultures of A1 and A2 strains generally yielded normal progeny, but atypical progeny were also recovered which appeared to be polyploids since all markers from the parents were present.