Technical Abstract: Cylindrocladium black rot (CBR), caused by Cylindrocladium parasiticum, and root-knot nematode, Meloidogyne arenaria, both infect and cause damage to the roots of peanut. Greenhouse and microplot experiments were conducted with the runner type peanut genotypes C724-19-15, C724-19-25 and Georgia-02C with different levels of resistance to nematode and CBR to better understand the interactions between the two pathogens. In the greenhouse, inoculation of 500-3000 eggs per plant of M. arenaria did not affect the level of root rot induced by 1.0 to 5.0 micorsclerotia of C. Parasiticum per g soil. In microplots, the root rot ratings from Georgia 02C and C724-19-25 were higher in plots infested with M. arenaria (0.4-2.0 eggs per cm3 soil) and C. parasiticum than in plots with C. parasiticum alone; however, M. arenria did not increase the root rot ratings on the nematode resistant C724-19-15. This was inconsistent with results in the greenhouse. Gall indices were not affected by C. parasiticum inoculations in the greenhouse or microplots. In both2006 and 2007, a significant interaction between C. parasiticum inoculum densities and nematode level was observed on plant mortality. CBR inoculum greatly increase mortality on C724-19-25 and Georgia-02C, but not on C724-19-15, in the presence of M. arenaria. The mortality increase was more apparent at lower levels of both pathogens, but on nematode-susceptible cultivars plant mortality was more with co-inoculations of two pathogens than from either alone. Simultaneous inoculation with the two pathogens decreased yield of C724-19-25 and Georgia-02C as C. parasiticum inoculum levels increased, but even the largest inoculum of M. arenaria (2.0 eggs per cm3 soil) did not decrease yield of C724-19-15.