The life cycles of 4 species of stonefly and 11 of mayfly are described, based upon the flight season of the adults and measurements of nymphs collected at all seasons from the Speed River. It is shown that they fit into all but one of the categories of life cycle that have been described for stream insects in Europe, and that where more than one species of the same genus coexist there is usually a marked difference in the timing of their life histories.