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Biologically produced molecular hydrogen (H-2) is characterised by a very strong depletion in deuterium. Although the biological source to the atmosphere is small compared to photochemical or combustion sources, it makes an important contribution to the global isotope budget of H-2. Large uncertainties exist in the quantification of the individual production and degradation processes that contribute to the atmospheric budget, and isotope measurements are a tool to distinguish the contributions from the different sources. Measurements of delta D from the various H-2 sources are scarce and for biologically produced H-2 only very few measurements exist. Here the first systematic study of the isotopic composition of biologically produced H-2 is presented. In a first set of experiments, we investigated delta D of H-2 produced in a biogas plant, covering different treatments of biogas production. In a second set of experiments, we investigated pure cultures of several H-2 producing microorganisms such as bacteria or green algae. A Keeling plot analysis provides a robust overall source signature of delta D = -712 parts per thousand (+/-13 parts per thousand) for the samples from the biogas reactor (at 38 degrees C, delta D-H2O = +73.4 parts per thousand), with a fractionation constant epsilon H-2-H2O of -689 parts per thousand (+/-20 parts per thousand) between H-2 and the water. The five experiments using pure culture samples from different microorganisms give a mean source signature of delta D = -728 parts per thousand (+/-28 parts per thousand), and a fractionation constant epsilon H-2-H2O of -711 parts per thousand (+/-34 parts per thousand) between H-2 and the water. The results confirm the massive deuterium depletion of biologically produced H-2 as was predicted by the calculation of the thermodynamic fractionation factors for hydrogen exchange between H-2 and water vapour. Systematic errors in the isotope scale are difficult to assess in the absence of international