Heterotrophic bacteria are thought to be important components of aquatic ecosystems in several ways. These bacteria remineralize organic materials and convert some organic material into bacterial biomass. We examined data from 70 studies in which estimates of production of heterotrophic bacterial biomass (bacterial production) were reported for fresh-and saltwater ecosystems. In sediments, bacterial production was sigdicantly (p <0.001), positively correlated to sediment organic C content. Systems which had hlgh rates of benthic primary production (such as coral reefs) had rates of bacterial production greater than those predicted by sediment organic C content alone. In the photic zone of lakes and the ocean, bacterial production was significantly correlated with planktonic primary production, chlorophyll a, or numbers of planktonic bacteria. For all planktonic systems analysed, bacterial production ranged from 0.4 to 150 pg C 1-' d-' and averaged 20 % (median 16.5 %) of planktonic primary production. On an area1 basis for the entire water column, bacterial production ranged from 118 to 2439 mg m-2 d-' and averaged 30 % (median 27 %) of water column primary production. Heterotrophic bacterial production is, thus, a large component of total secondary production and is roughly twice as large as the production of macrozooplankton for a given level of primary production.