The survival of Aeromonas salmonicida was determined in sediment-water containing laboratory microcosms. Survival was estimated by calculating the decimal reduction times (DRT) of colony forming ability (cDRT) on Coomassie brilliant blue tryptone soya agar (TSA) (CBBA). Two sources of sediment and water were used in these experiments. One source, an outflow of a freshwater lake, produced sediments that were acidic and rich in humic acids. The other, a local river, gave sediments that were neutral and low in humic acids. The decimal reduction times in microcosms constructed from the humic acid-rich materials were of the order of 0.3 days, whereas those determined in microcosm, composed of river-derived materials, were approximately 40 days. This 100-fold difference in the decimal reduction times was demonstrated to be a function of the sediment source rather than the source of the water. The estimates of survival of A. salmonicida outside its host represent a critical element in any assessment of the risk of furunculosis transmission. The demonstration here, that such survival is highly matrix dependent, demonstrates that it is unlikely that a single figure could be used as an estimate of such survival in any formal quantitative risk assessment process. (C) 2002 Elsevier Science B.V. All rights reserved.