Sting nematode (Belonolaimus longicaudatus) is recognized as a pathogen of cotton (Gossypium hirsutum), but the expected damage from a given population density of this nematode has not been determined. The objective of this study was to quantify the effects of increasing initial population densities (Pi) of B. longicaudatus on cotton yield and root mass. In a field plot study, nematicide application and cropping history were used to obtain a wide range of Pi values. Cotton yields were regressed on Pi density of B. longicaudatus to quantify yield losses in the field. In controlled environmental chambers, cotton was grown in soil infested with increasing Pi's of B. longicaudatus. After 40 days, root systems were collected, scanned on a desktop scanner, and root lengths were measured. Root lengths were regressed on inoculation density of B. longicaudatus to quantify reductions in the root systems. In the field, high Pi's (>100 nematodes/130 cm(3) of soil) reduced yields to near zero. In controlled environmental chamber studies, as few as 10 B. longicaudatus/130 cm(3) of soil caused a 39% reduction in fine cotton roots, and 60 B. longicaudatus/130 cm(3) of soil caused a 70% reduction. These results suggest that B. longicaudatus can cause significant damage to cotton at low population densities, whereas at higher densities crop failure can result.