Co-dominant microsatellite molecular markers for Phytophthora infestans were developed and their potential for monitoring the genetic variation in populations was demonstrated in the UK, across Europe and worldwide. Markers were developed according to two strategies. First, several thousand P. infestans expressed sequence tag (EST) and bacterial artificial chromosome (BAC) sequences were screened for the presence of simple sequence repeat (SSR) motifs, and, of these, 100 candidate loci were selected for further investigation. Primer pairs developed to these loci were tested against a panel of 10 P. infestans isolates and approximately 10% were shown to be polymorphic and therefore appropriate for further testing. Secondly, the construction and screening of a partial genomic library resulted in the development of one additional polymorphic marker. The resulting 12 SSR markers were converted to higher-throughput fluorescence-based assays and used in combination with two previously published markers to characterize a wider collection of 90 P. infestans isolates from the UK and six other countries. Several isolates from the closely related species P. mirabilis, P. ipomoea and P. phaseoli collected from around the world were also genotyped using these markers. Amongst the 90 isolates of P. infestans examined, considerable SSR diversity was observed, with 68 different genotypes and an average of 3·9 (range 2–9) alleles per locus. When other Phytophthora species were genotyped, all loci were successfully amplified and the majority were polymorphic, indicating their transferability for the potential study of other closely related taxa.