The effects of the extreme (30 +/- 2, 50 +/- 2, 90 +/- 2%) and a common (70 +/- 2%) relative humidity (RH) levels on functional response and fecundity of Neoseiulus californicus (McGregor) (Acari: Phytoseiidae) to Tetranychus urticae Koch (Acari: Tetranychidae) were studied at a constant temperature 25 +/- 1 degrees C and 16:8L:D photoperiod. In the experiments, 5, 10, 20, 40 and 80 T. urticae eggs were given to the predatory mite for 24 hours. According to logistic regression analysis, N. californicus showed a Type II functional response at all RH levels. The attack rate (a) and the handling time (Th) of Type II response were calculated by fitting Holling Disc Equation to the data. These two coefficients were varied depending on the RH. The highest and the shortest, a and Th values were determined to be 1.636 and 0.031 at 50 and 70% RHs, respectively. While the lowest average daily mean numbers of eggs laid by N. californicus was found to be 0.33 at 30% RH and five prey density, the highest value was determined to be 3.10 at 70% RH and 80 prey density. Although, performance of N. californicus was influenced by the extreme RHs, the predator had successfully continued predation and oviposition activities under those conditions. Therefore, the N. californicus population should be included in augmentative biological control programs in which considerable range of humidity fluctuations is expected.