Samples of mites were collected from cotton plants in several regions of São Paulo State. Monthly counsts were made in three consecutive crops, generally planted in October. The data obtained permitted to establish graphs illustrating the fluctuations of mite populations in the respective regions. Tetranychus (T.) urticae and Polyphagotarsonemus latus generally showed peak of population density in February independently of climatic conditions. Rainfall affected the population densities. In relatively rainy years Polyphagotarsonemus latus presented a high population density whereas the contrary occurred with all the other species studied. The populational level of the mite Mononychellus planki and of predator mites increased as the cotton plants reached the end of their cycle, showing a peak in April.