The toxicity of cadmium with regard to the vegetative reproduction of duckweed, Lemna gibba, grown in sterile culture, was determined. The EC50 was found to be 800 ppb. Duckweed grown in 2.24 ppm cadmium (supplied as cadmium nitrate) for 7 days accumulated 98.5% of the available cadmium from the growth medium. Plants that had been grown for 7 days in 2.24 ppm cadmium and control plants were fed to red swamp crayfish, Procambarus clarkii, for 14 days. The concentrations of cadmium were measured in hepatopancreata and muscles of crayfish on Day 0 and in crayfish fed duckweed grown in cadmium for 14 days. Accumulation of this metal in hepatopancreata increased 26-fold, i.e., 176.80 ppb on Day 0 to 4657.56 ppb on Day 14, and in muscles almost 7-fold, i.e., 6.75 ppb on Day 0 to 46.28 ppb on Day 14. Crayfish fed cadmium-containing duckweed demonstrated inhibition (55% after 14 days of feeding) of acetylcholinesterase activity in their central nervous tissue compared to crayfish fed cadmium-free duckweed. The ovarian index and total lipids content in the ovaries of crayfish fed cadmium-containing duckweed demonstrated significant increases on Day 14.