Solarization is an effective soil treatment against race 4 of Fusarium oxysporum f. sp. vasinfectum. Despite the lack of effective alternatives, solarization is rarely used in cotton because of its high cost. Use of solarization might be increased if soil temperatures could be used to predict reductions in pathogen inoculum levels, thereby ensuring high levels of efficacy. However, relationships between survival of race 4 chlamydospores, soil moisture, and temperatures typical of solarized soil are not known. Survival of culture- and plant-produced chlamydospores of race 4, incubated at 40°C in dry or moist environments, indicated the importance of moisture in determining spore survival. Mortality of spores from either source was low under dry conditions and much higher under moist conditions. A 6-week exposure of culture-produced chlamydospores to temperatures from 30 to 40°C under moist conditions indicated limited mortality at temperatures ≤35°C. However, most spores were eliminated by day 6 at 40°C. A second study using a moist environment indicated high mortality of spores by 5 weeks at 37°C or by 10 days at temperatures from 39 to 41°C. These results should serve as useful guides in efforts to develop solarization protocols that maximize effectiveness based on monitored soil temperatures.