Seasonal population changes in chironomid fauna of an urban concrete-lined flood control channel system were studied from Aug., 1974–Sept., 1976. Benthic samples from 9 stations along a 20-km stretch of the channel system were collected biweekly and air and water temperatures and rainfall were also recorded.Midge larvae of 7 genera were taken.Cricotopusspp.,Dicrotendipessp., andChironomusspp., in that order, were of quantitative importance; the other 4 genera collectively formed 0–7% of the total monthly-collected larvae.Cricotopusspp. were present year-round, forming>86% of the total midges collected and 65–99% of the monthly totals.Dicrotendipessp. (densest May–Nov.) were also present throughout the year, but their monthly populations ranged from<1–34% of the total.Chironomusspp. occurred from July–Nov., their density reaching up to 28% of the total midges taken then.The numerical increase in midges during the spring and summer coincided with warmer air and water during these periods. Two to 3 cm or more of rainfall in the area in a 24-h period drastically reduced larval populations, but within 2–4 wk, after a period of rainfall, they greatly increased.