Abstract: Eleven metalaxyl-resistant isolates were obtained from sensitive strain of Phytophthora nicotianae var.nicotianae on plates containing series concentration of metalaxyl.The stability and levels of resistance for all the resistant isolates were determined;the fitness of 3 low and 1 high level of resistant isolates was compared.The results showed that the occurrence frequency of the resistant isolates increased when metalaxyl concentration increased within the range from 2 μg/mL to 20 μg/mL; the resistance of all the resistant isolates could be maintained stably subculturing on OA plates;the mutants could be classified into two phenotypes of resistance: 10 low resistant isolates,whose resistance levels ranged from 3.55 to 13.9-fold with the average of 8.60,and 1 high resistant isolate,whose resistance level was 510-fold;the fitness index of the low resistant isolates of 20M-001,10M-003,10M-006 and the high resistant isolate of 10M-004,were 0.56,0.38,0.38 and 0.87,respectively;there was no correlation between the fitness index and the resistance level of the resistant isolates.