The viable heterotrophic bacteria of Lake Grasmere in inland Canterbury (43° 05 S, 171° 45 E) were studied. Numbers of bacteria in water above a spring were positively correlated with rainfall in the 7 days before sampling, but bacteria in open water and from over the macrophyteElodea canadensiswere not. There was some increase in the bacterial population in the water overE. canadensisin the autumn to winter months. Numbers of bacteria in the open water ranged from 55 to 1020 cells per millilitre, and were outnumbered by algae by a factor of between 4 and 34. The bacterial population tended to increase after blooms of zooplankton, but there was little or no response to changes in phytoplankton populations. Benthic bacteria rarely increased in numbers after the deposition of organic matter.