The Bacillariophyta dominated over the other phyla and were mainly recorded in high densities during summer and autumn. The Chlorophyta and Jlyxophyta (Cyanobacteria) were represented by coccoicl forms and by non-heterocystous, filamentous forms, respectively. Both were mainly recorded during summer and mostly absent during winter. Other phyla were occasionally recorded in low densities; the Chrysophyta being found in the River Elan and a t one site on the River Wye during June 1980. Their members were considered as "contaminants" or "fall-out'' from other communities. Pennate diatoms were the most "constant" species and either showed a general upstream or downstream increase or a general distribution throughout the study area. Populations of the same species colonising both sediments and stones were not correlated or insignificantly correlated with each other a t most stations. It was concluded that sediments s e r e unsuitable for algal colonisation. The River Elan and upper Wye were rated as oligosaprobic, the River Ithon and Ion-er Wye as 8-mesosaprobic and the River Lugg as a-mesosaprobic.