Sporangia germination of Phytophthora infestans isolates belonging to three clonal lineages (US-1, -7, and -8) was assessed at temperatures ranging from 10 to 25°C. At 10°C there were no significant differences in germination percents among US-1, -7, and -8. At 18 or 20°C US-7 and -8 had significantly lower germination percents than US-1. At 21, 24, or 25°C all clonal lineages had low germination percents. Sporangia of the US-7 and -8 lineages germinated more quickly at 15°C (P = 0.001) during the first 2 h than did the US-1 lineage. The incubation period (IP), lesion area (LA), and sporulation per unit of lesion area (SPU) of the isolates were assessed on inoculated detached leaflets of susceptible potato cv. Norchip kept at 10, 15, 20, or 25°C. In general, IP declined exponentially and LA increased exponentially with increasing temperatures. SPU had a quadratic shape, with the maximum at 15°C. Averaged over all temperatures, the US-7 lineage had the shortest IP (59.3 h compared to 66.4 h for US-1 [P = 0.012] and 71.7 h for US-8 [P = 0.026]). Again, averaged over all temperatures, the US-8 lineage had a larger LA (P = 0.030) than US-1. There was no significant difference between US-7 and -1 for LA. There were no significant differences among lineages in terms of SPU. These results indicate that clonal lineages differ from each other in epidemiological attributes, but the differences can be complex.