PLANKTONIC bacteria are a fundamental component of the organic carbon cycle in aquatic systems(1). Organic carbon consumption by planktonic bacteria is the sum of bacterial production (BP) and bacterial respiration (BR), IL is now estimated that. 30-60% of phytoplankton production (the amount of inorganic carbon fixed by phytoplankton photosynthesis, corrected far phytoplankton respiration) in marine and freshwater systems is processed by bacterial(1-3). These estimates of carbon flow through bacteria are conservative, however, because losses due to bacterial respiration are seldom directly measured(4,5). We report here that bacterial respiration is generally high, and tends to exceed phytoplankton net production in unproductive systems (less than 70 to 120 mu g carbon per litre per day). A large proportion of the world’s aquatic systems have phytoplankton productivities below this value(6). Bacterial growth efficiency (BGE) is the result of BP and BR[BGE = BP/(BR + BP)]. Comparisons of our models of bacterial respiration with published models of bacterial secondary production(1,7) show that bacterial growth efficiency must range fi om less than 10% to 25% in most freshwater and marine systems, well below the values commonly assumed in many current ecological models(1,2,8,9). The imbalance between bacterial respiration and phytoplankton production suggests that in unproductive aquatic systems, the biological system is a net source of CO2.