The effects of pentachloronitrobenzene (PCNB) on plant parasitic and free-living nematodes were studied in vitro and under greenhouse and field conditions. PCNB exerts a direct toxic effect on larvae of M. incognita (Kofoid and White) Chitwood, Helicotylenchus dihystera (Cobb) Sher, Hoplolaimus galeatus (Cobb) Sher, Rotylenchulus reniformis Linford and Oliveira, mononchoid and dorylaimoid nematodes and the microbivorous nematode Pelodera chitwoodi (Bassen) Dougherty. Toxic effects were most pronounced on mononchoid and dorylaimoid nematodes and on P. chitwoodi. Greenhouse studies indicated that PCNB can cause an increase or decrease in nematode numbers depending on the nematode species involved and the concentration of PCNB used. The nematode species most likely to increase in numbers when PCNB is applied are Pratylenchus scribeneri Steiner, P. brachyurus (Godfrey) Filip. and Schuur, Stek., H. dihystera, and H. galeatus. The effect on M. incognita, dorylaimoid, mononchoid and saproph
