Abstract: Phytophthora infestans (Mont.) de Bary belongs to Oomycetes and it causes the most destructive potato disease worldwide – late blight. It originates from Mexico but it has spread wherever potatoes are grown. P. infestans populations are diversified, sexual or asexual and their composition may be affected by climate changes. Mating type, mitochondrial haplotype, Simple Sequence Repeats (SSR) markers, sensitivity to metalaxyl and virulence were evaluated to monitor changes in Polish P. infestans population.Samples of potato leaflets with single late blight lesions were collected from fields located in three regions of Poland: Mlochów, Boguchwala and Siedlce, in three years 2010, 2011 and 2012. In the region of Mlochów intensively protected fields are dominating. There are mainly small gardens and experimental fields near Boguchwala. In Siedlce region early and starch potatoes are cultivated. Total number of isolates tested was 365. Mating type, mitochondrial haplotype and SSR were evaluated using a PCR method. Sensitivity to metalaxyl was tested on rye A agar media. Virulence was tested on detached potato leaflets.Polish P. infestans population is diverse. We did not observe major clonal lineages. A1 mating type (69%) and Ia mitochondrial haplotype (72.7%) dominated. Most of the isolates were sensitive to metalaxyl (66%). We noted differences in population composition between the regions which indicate that cultivation system has an impact on the population of P. infestans. No Label