Effect of temperature on developmental duration and demographic parameters of Tetranychus urticae Koch and Eotetranychus carpini borealis (Ewing) were investigated at five constant temperatures. Immature development for T. urticae was shorter than that of E. carpini borealis. A major difference in developmental time and fecundity rates between the two species was observed at 30°C, where T. urticae had the shortest developmental time and highest daily fecundity compared with E. carpini borealis. Developmental times for the two spider mite species fell within the range of developmental times reported for other spider mites. Life tables were constructed for both species. The intrinsic rate of increase for T. urticae ranged from 0.08 at 15°C to 0.321 at 30°C, whereas that for E. carpini borealis ranged from 0.065 at 15°C to 0.151 at 30°C. Temperature-development data were fitted to a nonlinear model. This model allowed the detection of some basic differences between the two spider mite species. In addition, simple linear regression was used to estimate lower developmental thresholds and degree-day requirements for development. Those data were in agreement with field observations.