WE have noted with interest recent reports which suggest a preferential biological availability of 238Pu over 239+240Pu in terrestrial and aquatic organisms1–4. Although kinetic isotope effects do occur in biological systems for low mass number elements (H, C, N), such effects are generally discounted as the mass number of the element increases. Thus, differential biological availability of isotopes of high mass number elements, such as plutonium, would normally be attributed to differences in the chemical or physical forms of the isotopes or to the different weights of isotope available to organisms as a result of large differences in their specific activities. These arguments have indeed been used to explain differential plutonium isotope behaviour in animals in controlled laboratory conditions5–7. It is not certain, however, that the same arguments can be used to explain anomalies of plutonium isotope behaviour in organisms contaminated by nuclear test debris or by wastes from nuclear fuel reprocessing plants. Plutonium activity levels, while easily measurable, do not approach those generally used in laboratory studies. In addition, geochemical weathering could conceivably alter any original differences in physical or chemical form of the plutonium. We report here the results of experiments which show that deposit-feeding marine worms living in sediments contaminated with plutonium isotopes in different ways do not preferentially accumulate 238Pu over 239+240Pu. This is particularly significant in that the worms held in sediments contaminated in the natural environment may have been exposed to different chemical and physical forms of plutonium isotopes.