Four sorghum lines of varying rust susceptibility, ranging from very resistant to susceptible, were planted in early September T985 in Isabela, Puerto Rico. Rust reactions were periodically evaluated from boot stage until grain harvest. At physiological maturity, foliar rust coverage was 24, 19, 7, and 0.5% for SC 212, SC 307, TAM 428, and SC 120, respectively. Four rust fungicide (oxycarboxin at 0.5 kg/ha/spray), applications at and after boot stage resulted m final rust coverage of 7, 3, 1, and 0% on SC 212, SC 307, TAM 428, and SC 120, respectively. Over all cultivars, one, two, and four applications gave 32.9, 73.7, and 85.4% rust control compared to the nontreated plots. In moderately and very resistant varieties (TAM 428 and SC 120), rust control was greater (74.5%) than that found in moderately and very susceptible ones (55.2% for SC 307 and SC 212). On SC 307 and SC 212, yield losses from rust were 29 and 50%, respectively. Reductions in 100-seed weights in SC 307 and SC 22 (28 and 41 %, respectively) approximated yield loss levels. Yield of SC 212 was superior to that of SC 307 when rust was controlled with oxycarboxin and inferior to that of SX 307 when rust was untreated. Moderately rust resistant TAM 428 showed no response to rust control despite 7% rust coverage. This same rust level reduced yield of the susceptible varieties. Because of the genotypic differences in rust-yield reactions, a generalized model relating visual rust coverage to plant performance appears inappropriate.