The diurnal and seasonal activity patterns of the western cherry fruit fly, Rhagoletis indifferens Curran, were determined on three sweet cherry trees, Prunus avium (L.), from 0700 to 1900 hours during June and July 2001 in Yakima County, WA. There were significant effects of time of day and season on numbers of flies seen on fruit relative to leaves, indicating that presence on or attraction to these substrates for feeding, mating, or oviposition, whether caused by visual or chemical cues, was greatly modified by temperature. When temperatures were 15–16°C at 0700 hours in early June, flies of both sexes were seen mostly on leaves, but when the temperature reached 20°C, more flies of both sexes moved onto fruit. Peak fly sightings occurred at 25–35°C, usually after 1000 hours. At >35°C, most flies vanished from view, and those seen were less active. Over the entire season, males were seen four times more often than females and spent more time on fruit than on leaves. Females were seen equally on and spent equal time on fruit and leaves when temperatures were 20–35°C. Mating was initiated on fruit but was completed on fruit and leaves. Overall fly sightings and daily temperatures were positively correlated early in the season when it was cooler, uncorrelated in the middle of the season, and negatively correlated in late season when it was warmest. The results show that R. indifferens presence and times spent on fruit and leaves are greatly altered by daily and seasonal changes in temperature, but they also suggest that at 20–35°C substrate-seeking behaviors of each sex do not change with time of day or season.