Cedix, a cultivar of cotton, Gossypium hirsutum L. from El Salvador, is highly resistant to the cotton leaf crumple virus (CLCV), which is transmitted by the sweetpotato whitefly, Bemisia tabaci (Genn.). In 1987 and, 1988, Cedix was crossed to the highly susceptible Deltapine 90, a preferred cultivar in southwestern desert cotton production. Following a 7-day exposure of 2-week old seedlings of the two parents, the F₁, F₂, and backcross generations to viruliferous whiteflies under greenhouse conditions (in April 1988 and 1989), the inoculated seedlings were transplanted to the field. Plants were scored several times during the two growing seasons for symptoms of cotton leaf crumple disease (CLC). The results were similar for both the 1988 and , "89 experiments. All plants of Deltapine 90 showed -- severe symptoms, and all plants of Cedix, as well as those of the F₁, were apparently asymptomatic. In the F₂, and the backcross to Deltapine 90, the typical severe-symptom phenotype was the only unequivocal phenotype; it was difficult to distinguish a mild expression of CLC symptoms from the asymptomatic expression. The F₂ segregated 1 severe: 15 "other", the backcross from F₁ X Deltapine 90 segregated 1 severe: 3 "other", and the backcross from F₁ X Cedix had no plants with severe symptoms. Thus a 2-gene model is suggested, with resistance dominant to susceptibility (or partially dominant, because some F₂ and backcross plants show mild CLC symptoms). Testing of individual progeny will be necessary to select true-breeding resistant plants from a segregating population. A population of [Deltapine 90 X (Deltapine 90 X Cedix)] BCzF2 segregated 27% severe-symptom plants when exposed to natural whitefly populations at Yuma, AZ. This result suggests that possibly one of the resistance genes was lost in the process of selection and backcrossing to Deltapine 90. The F₂ populations from Cedix crossed with four other cultivars, and from the CLCV-resistant, nectariless 'Conal' crossed with four germplasm lines, segregated from 0 to 31% severe-symptom plants.