The life history of the carmine spider mite, Tetranychus cinnabarinus (Boisduval) (Acari: Tetranychidae) was studied under conditions of 4 constant temperatures between 19°C and 35°C, and 6 relative humidities, from 0 % to saturation. More eggs were deposited at 24°C and 38 % RH than at any other combination of temperatures and relative humidities, and least preadult mortality occurred under these conditions as well as at 30°C and 38 % RH or 63 % RH. Very high relative humidities retarded development of all mite stages at all temperatures, no adults being produced under saturation conditions. The fastest development, accompanied by much mortality, occurred at 35°C and 38 % RH. Life tables were constructed from these data utilizing a simplified method for computing the intrinsic rate of increase (rm)· The greatest net multiplication rate per generation (R0 ) is at 24°C and 38 % RH. The bearing of these results on the cultural control of some spider mite populations by very high humidities is discussed.