After a long course over a stony substratum, the River Lune, England, runs over flat rock covered with moss. Larvae of four species of Ephemeroptera are abundant in this moss‐covered area from spring to autumn. They overwinter in the egg stage.Baetis rhodaniabundant in much of the river at all times of year, is numerous in this stretch in winter but absent from most summer collections. This absence is tentatively attributed to exclusion by the summer species.