Collections of biting Leptoconops americanus were made at half-hour intervals throughout the daylight hours on Stansbury Island, UT, during 9 days in May, 1993. The most favorable conditions for biting (> or = 90 bites on the ears in 15 min) included temperatures higher than 15 degrees C, minimum wind (< 5 kph), minimum cloud cover, maximum sun, and no rain. Temperatures below 10 degrees C or the presence of rain prevented almost all biting. Higher winds and cloudiness decreased biting activity, but did not eliminate it if other conditions were favorable. Although not statistically significant, there was some suggestion from the data that higher temperatures (> 25 degrees C) reduced biting. The flies did not appear to be more numerous at any particular part of the day; the biting rate simply followed meteorological conditions at the time. Ambient light varied between 1 and 10,000 foot candles during the study, with high biting rates (76 and 99 bites per 15 min) observed at levels as low as 80-100 foot candles.