Aerobic heterotrophic bacteria were isolated from the water, mud,Elodea canadensis, and concentrated plankton of Lake Grasmere in inland Canterbury (43° 05’ S, 171° 45'E) at intervals between April 1969 and July 1971. The characteristics of 1567 isolates are described. The bacteria were assigned to the following groups: pseudomonads, pink‐pigmented pseudomonads,Alcaligenes/Achromobacter, flavobacteria,Cytophaga, Enterobacteriaceae,Aeromonas/Vibrio, Acinetobacter, Chromobacterium, coryneforms, Micrococcaceae, andBacillus. In the water the predominant bacteria in most of the samples were pseudomonads,Alcaligenes/Achromobacter, flavobacteria, and coryneforms. The bacterial population varied from year to year but at any one time the bacteria isolated from different areas of the lake were similar. Three mud samples were examined: only 20% of the bacteria isolated from a sample from the surface of the mud were Gram‐positive, but the other two samples from up to 5 cm deep in the mud included 49% and 74% Gram‐positive bacteria.Elodea canadensissupported an epiphytic population, predominantly of pseudomonads and flavobacteria, which was more stable than the population in the water. Enterobacteriaceae andAeromonas/Vibrio, although common at times in the water, were rarely isolated fromE. canadensis. Bacteria were enumerated from samples of concentrated plankton as the numbers of algae and zooplankton fluctuated. Less than 17% of the colony‐forming heterotrophic bacteria were in close association with the plankton. No particular group of bacteria became more common as the plankton populations died back.