Effects of black plastic mulch and clear plastic canopy on the nematode populations and southern blight of tomato were investigated at the University Field Station, Trinidad. The results showed significant differences (p = 0.05) in the effectiveness of the various treatments against populations of Rotylenchulus reniforrnis, Aphelenchus avenae, Meloidogyne incognita and Tylenchus sp. The incidence of southern blight (Sclerotium rolfsii) was reduced in the black plastic mulch and mulch plus clear plastic canopy treatments but greatly increased in the clear plastic canopy treatment when compared wich the control. Much of the reduction in yield of agricultural crops in the Eastern Caribbean has been partly attributed to plant parasitic nematodes either directly or in association with other soil borne disease-causing organisms e.g. Fusarium, Sclerotium rolfsii Sacc. and Pseudomonas solanacearum Sm, (1, 2, 7). The conventional practice of vegetable crop rotation and other cultural practices e.g. fallowing, occasional flooding and use of organic manure do not seem to be effective in preventing the build-up of plant pathogens. Soil fumigants have been found to be effective in significantly reducing nematode populations (11) but because these fumigants are expensive their use is restricted to high value crops. The effects of plastic mulch on the nematode populations and on southern blight have not been investigated in the Eastern Caribbean. Sandhu and Dalai (10) recently reported the effects of black plastic mulch and clear plastic canopy on the soil properties and their relationship to tomato yield in low-land tropics. The beneficial effects of plastic mulches and/or clear plastic canopies have been reported by other workers (4, 6, 9). The purpose of this study was, therefore, to investigate the effect of black plastic (polyethylene) mulch and clear plastic canopy on nematode population and southern blight of tomato.