Abstract: In 2003 and 2004, 127 Phytophthora infestans isolates were sampled from Inner Mongolia and Heilongjiang, China, their sensitivities to metalaxyl and cymoxanil were determined in vitro. The results showed that the metalaxyl-sensitive(MS) isolated in 2003 from Inner Mongolia, 2003 and 2004 from Heilongjiang, covered 86.2%, 13.8% and 17.8%,respectively. Although cymoxanil has been used nearly for 10 years in these areas, but no any decreasing in sensitivity of P.infestans populations has been detected. Their EC50 values for cymoxanil ranged from 0.10 μg/mL to 0.35 μg/mL,the mean EC50 value (±SD) was 0.20(±0.06) μg/mL exhibited unimodal distribution of sensitivity, which could be used as the baseline-sensitivity for monitoring the field resistance development in the future. There was no cross-resistance between metalaxyl and cymoxanil in field populations of P.infestans. Cymoxanil gave equal effect on both metalaxyl-resistant and wild-sensitive isolates.