The results of a 4-yr study are reported. Data from Surber-samples of 3 running waters in southern France were compared with results from field enclosure experiments. Egg development lasted about one month under field conditions and two weeks at 25°C in the laboratory. O. uncatus has usually but not strictly 13 larval instars and a 3-yr life cycle. Each year emergence started in early June. The emergence curve was typical of a summer sp. Several aspects of seasonal regulation are discussed. The monitoring of population structure indicated a high mortality of final instar larvae (up to 99%) during winter. This mortality differed between the sample sites of one of the running waters. These differences depend on prey availability and on larval density. A field enclosure experiment revealed density dependence of larval size and growth.