A greenhouse study indicated that high levels of soil K could not overcome the deleterious effect of high reniform nematode (Rotylenchulus reniformis) populations on cotton yield and maturity. Reniform nematodes and K fertilization independently affected plant height and weight but did not affect root mass. Reniform nematodes delayed crop maturity by as much as 2 to 3 weeks, but had no effect on K concentration in whole plants. There was no interaction between level of K fertilization and reniform nematodes on any of the cotton growth parameters measured. Reniform nematodes suppressed root-knot nematodes.