Water samples were collected each month from seven sites in the Clyde estuary, Scotland, throughout 1990. The numbers of naked amoebae in each sample were enumerated following enrichment cultivation. Amoebae showed a seasonal trend with greatest abundances over the summer months, June and July. Numbers throughout the year ranged between 0 (below detection) to 43·0 amoebae ml−1 with an overall annual mean of 8·3 amoebae ml−1. A total of 21 different morphotypes were identified and some of these had locomotive forms less than 10m in length. These small forms, in particular, constitute a previously overlooked group of protists. Our data suggest that amoebae were frequently associated with suspended floc material where they were uniquely suited to graze the absorbed floc bacteria and other microbes. There were occasions throughout the year when the numerical density of amoebae equalled that of ciliates, thus they are probably contributing significantly to the cycling of estuarine carbon and deserve further consideration.