The daily patterns of activity of females of the orange wheat blossom midge, Sitodiplosis mosellana (Géhin), were observed in controlled laboratory conditions and in field conditions in eastern Saskatchewan in mid-July 1986 and 1987. In the field, during the daytime, females rested on stems of wheat plants within 30 cm of the ground and, at approximately 2000 hours CST, flew up to wheat heads. Most oviposition took place between 2000 and 2145 hours (or 75 min before, to 30 min after, sunset). Toward the end of the oviposition period, females were frequently seen drinking dew from wheat heads. On some evenings, females migrated down from the heads following oviposition, but on more than half of the evenings they remained on the heads until early morning. However, they never moved down to the low level they occupied during the day until the next morning, when the migration was usually complete by 0900–1000 hours. Light intensity appeared to regulate the vertical migration of females. Cloudy conditions may allow an earlier onset of oviposition. Flight was limited to air temperatures above 14–15 °C and oviposition to temperatures above 10–11 °C. Wind speeds of 10 km per h or more and rain occasionally limited activity. In the laboratory, oviposition activity occurred almost exclusively during the scotophase, mainly in the first 2 h. Mean total fecundity was 83.6 ± 10.9 (±SE) eggs, and mean longevity was 6.6 ± 0.6 days. No oviposition took place on the 1st night, and the greatest mean daily fecundity occurred on the 3rd night.