Three species of freshwater tortoise, Emydura macquarii, Chelodina longicollis and C. expansa, occur in the River Murray. Previous evidence predicts that the populations of tortoises in the Murray may be in decline due to predation on nests by introduced foxes. This paper attempts to estimate the total biomass and energy flow-through of tortoises in the Murray in an effort to assess the possible consequences of widespread decline in the populations. I estimate that, in the Murray from upstream of Lake Hume to the Barrages, there are between 10 000 and 100 000 tonnes of tortoises. It is not possible to make more accurate estimates with the limited data presently available. The energy through-put of tortoises in the Murray, therefore, is between 13.5 × 10⁷ and 13.5 × 10⁸ kJ/day. The two most abundant species, and possibly all three, consume considerable quantities of carrion. By assuming that their diet includes 20 per cent carrion, as much as 430 tonnes of carrion may be consumed by tortoises each day in summer. The disappearance of tortoises from this system could, therefore, potentially result in a considerable problem with eutrophication. Estimation of the quantitative importance of tortoises in the system awaits further detailed research.