Somatic recombination is the exchange of nuclei or cytoplasm between two fungal isolates without the need for any specialised sexual structures and its detection requires the use of phenotypic or nuclear markers. Previous studies using virulence or drug-resistant markers suggested that somatic recombination occurred between some isolates of P. infestans, but recombinants could not be identified from mutants without molecular markers. The aims of the study were to evaluate the usefulness of AFLPs for detecting variation in P. infestans and then use the AFLP technique to identify somatic recombination in self-fertile (Al/A2) and self-sterile (Al/Al) pairings. The ability of Restriction Fragment Length Polymorphisms (RFLPs) (probe RG57) and Amplified Fragment Length Polymorphisms (AFLPs) to detect polymorphisms in Phytophthora infestans was compared. Ninety-eight isolates, representing 35 RG57 fingerprint patterns and mitochondrial haplotypes Ia and Ila were fingerprinted for AFLPs. UPGMA analysis of AFLP and RG57 fingerprints suggested that sexual reproduction may be uncommon or isolated in P. infestans populations in the UK as isolates of A2 mating type or mitochondrial haplotype Ila had similar fingerprints. Some isolates with the same RG57 fingerprint had quite different AFLP fingerprints, suggesting convergence of the RG57 fingerprint. Somatic recombination was studied using various approaches. Asexual progeny from three self-fertile isolates ( one synthesised in the lab with known parents, the others isolated from the field) were investigated. No evidence was obtained in support of somatic recombination in the synthesised self-fertile using the RG57 probe; the 180 hyphal-tips and single-sporangia isolated from one self-fertile all had the fingerprint pattern of one parent. Twenty-six of the 92 progeny fingerprinted for AFLPs had non-parental AFLP fingerprints. This and further AFLP variation from two field self-fertiles could have been the result of mitotic crossing-over or somatic recombination. Forty-three pairings were constructed (mainly AlxAl) with different drug-resistant markers (metalaxyl, streptomycin, geneticin or hygromycin). Progeny from two pairings exhibited a combination of parental phenotypes but these were unstable and quickly lost their ability to grow on double-drug amended media. Protoplast fusion between drug-resistant strains yielded some hyphal-tip or single-sporangial lines able to grow on double-drug amended media. Single-zoospores isolated from these strains displayed the double-drug phenotype and also contained AFLP bands from both parents, suggesting that karyogamy had occurred in these pairings. Co-cultivating two A 1 isolates on leaves, allowed single-sporangia to be selected with race phenotypes appearing as a combination of the two Al strains. AFLP analysis of these strains suggested that these apparent "recombinants" were possibly the result of mutation in one of the parental isolates. The AFLP technique proved to be useful in identifying somatic recombinants in protoplast fusions. The lack of definite evidence for somatic recombination in other pairings is discussed.