Genetic mechanisms of increase, persistence, decrease and decline of major acaricide resistance in citrus red mite, Panonychus citri (McG.), were studied. Dicofol resistance in P. citri was mainly due to a single, incompletely recessive major gene. The result of the experiments on fitness indicated that the reproduction rate of the resistant strain was lower than that of the susceptible one, in particular under a high temperature condition (30℃) or under a special feeding condition such as seriously damaged leaves of host trees. This was caused by a lower rate of egg production in the resistant mites. The dicofol susceptibility in a mite population originally composed of a 1 : 1 mixture of resistant and susceptible strains from the same origin was examined in 17 generations at 25℃. After 13 generations, susceptibility of the mixed population increased remarkably. The result indicates that the resistant mites (RR) have lower fitness values than the susceptible mites (SS, RS) under either favorable (25℃) or unfavorable conditions for reproduction of the spider mites in an acaricide-free environment. Amitraz resistance was due to a single, incompletely dominant major gene. The values of fitness of benzomate and amitraz resistant mites were almost equal to those of susceptible mites in an acaricide-free environment.