A survey of Windermere North and South Basins and Esthwaite Water at two sites at the south end of the lake was made in an attempt to establish the degree of association between the phytoplankton and the bacteria. Positive correlations were established, when all samples were considered, between chlorophyll a estimates and bacterial counts. This association varied with the species involved and the degree of attachment by bacteria was seen to diminish along the series colonial green and blue-green algae > filamentous green and blue-green algae > diatoms > dinoflagellates. This trend was observed when using `viable' as well as direct counting methods. An increase in the degree of attachment to moribund algal cells when compared with healthy cells was also noted. The positive correlation of bacterial estimates (`viable' and direct counts) with water temperature was observed when all samples were included in the analysis. Alkaline phosphatase activity of paper-filtered samples correlated positively with both algal and bacterial numbers; activity of membrane-filtered samples was significantly correlated only with numbers of phosphatase-producing bacteria. Further examination is required before the origin and control of the enzymes can be established. Estimates of algal and bacterial biomass were made in an attempt to determine their respective roles in the CO2 balance of the lakes. It was noted that indirect estimates of bacterial CO2 evolution were considerably lower than similar estimates of the net uptake of the algal biomass.