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Randomly selected clones from a Phytophthora infestans partial genomic library were characterized by hybridizing individual clones to Southern blots of total genomic DNA digested with the restriction enzyme EcoRI. Among 59 clones that were screened on seven different central-Mexican isolates, five revealed a unique banding pattern for each isolate tested. Two of these clones were tested further; the banding patterns produced by both were somatically stable when probed to DNA from 63 single-zoospore (asexual) progeny from five different “parent” isolates. For one probe, RG57, each band appeared to represent a unique genetic locus in three different crosses, and each locus segregated for the presence or absence of a band. No bands were found to be allelic, but two pairs of cosegregating loci were identified. Genetic analyses of the other probe (RG7) revealed many more pairs of cosegregating …