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Abstract: The stabilising effects of natural benthic diatom and bacterial assemblages on cohesive sediments were compared with those caused by physico-chemical binding alone. Cohesive sediment beds were reconstructed in 4 annular laboratory miniflumes, using sediment collected at 5–6m water depth from a local fjord. The sediment was left to stabilise (consolidate) for 1, 2, 5 and 10 days, before being fully resuspended in a series of erosion experiments. The flumes were aerated and subjected to different light/dark conditions; antibiotics were used to isolate diatom from bacteria effects. During consolidation, a constant current velocity was maintained, at a speed well below erosion threshold. ‘Natural’ sediment regained 27% of its original stability after 1 day, and 85% after 5 days. Complete ‘natural’ sediment stability was regained within 10 days. Benthic diatoms (mainly Nitzchia sp., Gyrosigma sp. and Pennales sp.) were responsible for about 80% of the biostabilisation, whilst bacteria contributed ∼12%, indicating the importance of light as a controlling factor for surface sediment stability. Relative to physico-chemical binding, the increase in erosion threshold induced by benthic diatoms was 120%, and by bacteria 20%. Where the assemblage consisted of both diatoms and bacteria, the increase was 150%, indicating that natural bed values are not a simple summation of the two effects, i.e. diatoms plus bacteria. Using the results, a first generation empirical relationship describing the relative contributions of microbenthic and physico-chemical factors in the development of erosion threshold is presented. Empirical relationships containing biostabilisation coefficients are important for better computer model predictions of sediment transport. [Copyright &y& Elsevier]