In many tropical regions, cassava is a major food source, especially for subsistence farmers. It is high in carbohydrates and very productive in semi-arid regions and poor, acid soils. Production of cassava has consistently increased in recent years, but post-harvest deterioration continues as a major obstacle to its greater commercialization. Over 40 introduced and local cultivars were planted at Isabela, Puerto Rico, to determine post-harvest losses as influenced by fungicide treatment, cultivar, and storage time. Root deterioration was rated at four and 16 days after harvest using the CIAT scale. Hydrogen cyanide (HCN} content was determined at harvest; and dry matter content and culinary quality were measured at harvest and on the two-post harvest dates. Before storage, roots were either not treated or submerged in 4,000 ppm thiabendazole in aqueous solution. Some of the introduced cultivars showed superior storage and culinary qualities compared to the Puerto Rican commercial cultivars evaluated. Resistant cultivars and fungicide treatment were highly effective in delaying post-harvest deterioration. When treated with fungicide, 10 cultivars had maintained less that 5% root deterioration at 16 days after harvest. Pl 12903 had outstanding culinary quality even after 16 days' storage. Only Brava and Pl 9608 contained dangerously high levels of HCN, exceeding 115 ppm.