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The spittlebug, Aeneolania albofasciata Lalleman, is the most economically important insect pest of buffelgrass pastures in Mexico. In 1984-1986 we studied the effect of climate on the life cycle of this insect in Sonora, Mexico, by monitoring spittlebug nymph and adult densities in relation to summer precipitation and maximum and minimum temperatures. The amount of summer precipitation appeared to be the most important climatic factor affecting spittlebug population density and duration of nymphal and adult life stages. The correlation coefficient between cumulative summer rainfall and density of nymphs was 0.93, and for adults it was 0.92 in the 3 yr. Mean maximum, and minimum temperatures, however, were not related to adult or nymphal densities in any year. Hatch of overwintered eggs was initiated after 45-60 mm of summer rainfall. Total and peak adults were higher in 1984 when precipitation was two times above normal (320 mm) than in 19S5 and 1986 when precipitation was below and approximately average. Insect densities declined when preoipitation was below normal (286 mm) in 1985. Life-cycle duration from the first-instar nymph through adult averaged 43 +/- 3 d across the 3 yr. The spittlebug was univoltine in 1985 and 1986, however, during wet years (such as 1984), nymphal and adult stages can be present for a longer period.