[en] The production and use of the photon-emitting isotope plutonium-237 in investigations of the uptake, retention, and distribution of monomeric plutonium (IV) both in an aquatic vertebrate, the channel catfish Ictalurus punctatus, and in a littoral aquatic micro-ecosystem, are presented. The rationale for use of plutonium-237 in environmental studies is discussed. Chelation can either enhance or reduce the uptake of ingested plutonium relative to plutonium hydroxide (monomer) in channel catfish. The highest observed retention (whole body) at 63 days was 3.8% of ingested dose for plutonium-237 citrate, while retention of the fulvate was 0.6%. Reduced uptake of the fulvate complex is due either to its high molecular weight (>10,000) or to its stability in metabolic systems. Increased uptake of plutonium-237 citrate is attributable to instability of the complex in metabolic systems. Tissue distribution studies revealed that relatively little (less than or equal to 10%) of intracardially injected plutonium citrate was excreted. Blood clearance rates were similar to those found in small mammals, with the plutonium being primarily associated with the plasma protein transferrin. The fractional body burdens in bone, liver, and kidney 17 days after injection were 31%, 24%, and 9% of the injection dose, respectively. High kidney burdens relativity to mammals are expected, since the kidney functions as the major site of homopoiesis in teleosts. Absence of significant excretion indicates that a short half-life component of elimination following gut clearance in gavage studies is due to plutonium labeling of the gut