Isolates from potato in an isolated garden in North Wales in 1995 consisted of essentially two distinct phenotypes. Phenotypes P1 and P2, of A1 and A2 mating type respectively, had distinctive multi-locus (RG57) and telomeric RFLP fingerprints. In addition, about 6% of isolates forming abundant oospores were self-fertile and showed a combination of both fingerprints. Among a total of 40 single-hyphal-tip and single-sporangial propagations from two self-fertile isolates, 16 were P2 whereas only one propagation was P1; four propagations were still self-fertile and still carried the molecular markers typical of both P1 and P2. Most of the remainder were A1 but otherwise carried both P1 and P2 molecular markers. Uninucleate zoospore lines from two of these were also of A1 mating type but had segregated molecular markers typical of P2; a zoospore line from a third segregated both A2 mating type and markers typical of P2. The original self-fertile isolates appeared to have heterokaryotic hyphae and sporangia with P1 and P2 nuclei and also nuclei of A1 mating type but P2 markers. Tetraploid nuclei within a heterokaryon, resulting from fusion of P1 with P2 nuclei, could have segregated to yield the latter.