Late blight in potato is caused by the fungus Phytophthora infestans and can inflict severe damage on the potato crop. Resistance to P. infestans is either based on major dominant R genes conferring vertical, race-specific resistance or on "minor" genes inducing horizontal, unspecific resistance. A dihaploid potato line was identified which carried the R1 gene, conferring vertical resistance to all P. infestans races, with the exception of those homozygous for the recessive virulence allele of the locus V1. The F1 progeny of a cross between this resistant parent P(R1) and P(r), a line susceptible to all races, was analysed for segregation of R1 and of restriction fragment length polymorphism (RFLP) markers distributed on the potato RFLP map comprising more than 300 loci. The R1 locus was mapped to chromosome V in the interval between RFLP markers GP21 and GP179. The map position of R1 was found to be very similar to the one of Rx2, a dominant locus inducing extreme resistance to potato virus X.