Growing cotton in a greenhouse with 12-h of supplemental light [8,608 lux (800 ft-c) from combination of mercury and Lucalux lamps] resulted in 2 x to > 3 x greater reproduction of Meloidogyne incognita and Belonolaimus longicaudatus as compared to natural light alone. Rate of increase of Hoplolaimus galeatus was affected little in this experiment. In a second experiment under controlled conditions in a phytotron, light source and intensity had greater influence on the reproduction of Heterodera glycines and Pratylenchus penetrans on soybean than on B. longicaudatus. Fluorescent plus incandescent and metal halide light sources resulted in the greatest nematode reproduction. Lucalux lamps resulted in much lower rates of nematode increase than other light sources. Rates of nematode increase on soybean under the different light sources in the phytotron generally were positively related to plant growth.