Polyphosphate (poly-P) was detected with the use of 31 P nuclear magnetic resonance (NMR) spectroscopy in sediments from a large variety of lakes with different trophic state and morphometry. In the top 0.5 cm of sediment, poly-P was 1.5 to 11.4% of total P. Nonreactive phosphorus (NRP) in the NaOH fraction (often classified as organically bound phosphate) was up to 46% inorganic poly-P. In some surface sediments, the poly-P content equalled the iron-fixed phosphorus determined by chemical phosphorus fractionation. Sediments were probably supplied with poly-P by sedimentation because there were substantial amounts of poly-P in plankton and settling seston. As demonstrated with sediments of Lake Petersdorf, benthic organisms can also contribute to the formation of poly-P (up to 0.11 mg P [g dry weight]Ϫ1 ) under favorable aerobic conditions. Poly-P is more rapidly transformed into single orthophosphate during diagenesis than other inorganic and organic P species. The transformation of organic P compounds and poly-P can contribute significantly to the release of P during diagenesis and should be considered along with the reductive dissolution of P sorbed to iron oxihydroxides.Microbially induced changes in pH and redox potential affect the ability of lake sediments to retain inorganic phosphorus (Roden and Edmonds 1997; Gächter and Müller 2003). Although these changes are driven by organic matter decomposition, the direct role of sediment bacteria by the release of organically bound phosphorus often has been ignored. Many studies have shown that a substantial proportion of phosphorus in settling seston originates from biomass (e.g., Hupfer et al. 1995a;Pettersson 2001;Kleeberg 2002). The decreasing content of these organic P species in settled particles during early benthic diagenesis indicates that heterotrophic sediment bacteria mineralize sedimentary organic P to inorganic P (Wetzel 1999). However, some of the supplied P is assimilated during microbial growth. Studies with activated sludge and with mixed or pure cultures have shown that several types of microorganisms are able to take up P excessively and form intracellular polyphosphate (poly-P; Wentzel et al. 1991). In waste water treatment plants with biological P elimination, poly-P-accumulating organisms are dominant under oscillating redox conditions. AcknowledgmentsWe thank Christiane Herzog for her help with the analytical work. René Gächter, Roland Psenner, Jörg Lewandowski, and two anonymous reviewers are acknowledged for critical reading and helpful comments on a former version of the manuscript. Sarah Poynton is acknowledged for the linguistic improvements of the text.