In revising the definition of high level radioactive waste unsuitable for dumping at sea, calculations of water concentrations in and near a dump site have been made and used to estimate doses to ‘typical’ marine species living at or near the sea floor at a depth of 4000 m. These calculations show that there are radionuclides that can give rise to significant doses to these typical species and that future revisions of the Definition and Recommendations under the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter will have to consider impacts on the marine ecosystem in setting limits for dumping. Contents: 1. Introduction; 2. Nature of the deep sea ecosystem; 3. Naturally occurring and fallout radionuclides in the marine environment; 4. Effects of radiation on aquatic organisms; 5. Methods of dosimetry for aquatic organisms and an estimate of the background dose rate in the deep sea; 6. Derivation of dose rates to organisms based on the IAEA definition of high level radioactive waste unsuitable for dumping at sea; 7. Dose rates arising from actual sea dumping operations; 8. Summary and conclusions; Appendix: Equations and parameters used for calculations and results.