1. The activity of Ecdyonurus torrentis Kimm., Heptagenia lateralis (Curt.) and Baetis rhodani (Pict.) has been measured over a series of 24 hr. periods by recording their movements over 10 min. periods every hour.

2. Diurnal rhythms have been found to occur under conditions of natural daylight and darkness in all three species, but the periods of activity occur at different times, according to the species.

3. These rhythms are maintained under conditions of continuous light or darkness and under conditions of ‘reversed’ illumination.

4. In Ecdyonurus torrentis the rhythm is not seen in nymphs bred from eggs in continuous light, but appears abruptly, in almost fully developed form, after exposure of the nymphs to natural daylight and darkness for a period of no more than 24 hr.

Now at Department of Zoology, University of Cambridge.