Fifty-eight isolates of Phytophthora infestans were evaluated in vitro and on detached leaves of potato for their sensitivities to metalaxyl-M and cymoxanil. The isolates belonged to the clonal lineage, EC-1, which is dominant on potato in Peru and Ecuador. All isolates were collected in Huánuco, Peru, an area of year-round potato production, where the potential for development of fungicide resistance is high. All isolates were resistant to metalaxyl-M, with in vitro EC50 values ranging from 468.30-813.57 mg l-1. In contrast, we found no evidence for resistance to cymoxanil for which in vitro EC50 values ranged from 0.03-1.11 mg l-1. Resistance to each fungicide was also evaluated for five isolates in a detached leaf assay in which the fungicide was sprayed on the leaf surface prior to inoculation. With metalaxyl-M, the range of EC50 values was 158.85-828.29 mg l-1, similar to that for the in vitro assay. For cymoxanil, EC50 values ranged from 1.41 to 2.31 mg l-1, which was higher than in the in vitro assay but still two orders of magnitude lower than the concentration applied by farmers in the field. © KNPV 2009.