Properties of nuclear debris that affect its distribution and concentration in water systems and the biological availability of radionuclides to edible aquatic organisms were assessed for debris from nuclear cratering events. From particle size distribution, it was calculated that longdistance transport of debris can occur in dynamic water systems. Leachability of debris was found to vary with the radionuclide and with the composition of the leaching solution, and to depend on particle size and distribution of activity in the particle. Biological availability was studied in aquaria or large pool systems where radionuclide concentrations were followed in water, suspended material, and aquatic organisms. Bivalve molluscs and crustaceans usually had higher radionuclide concentrations than fish. The availability of some radionuclides was less than that observed in experiments with pure, commercially obtained radionuclides and may be due to differences in physical and chemical form. Estimates of the whole-body dose to man from consumption of the aquatic organisms indicate that 131I, 140Ba/140La, 110mAg, and 181W are potential major contributors.