Suppression of symptoms of aster yellows by antibiotics supports the tentative hypothesis that the etiologic agent is a mycoplasma-or bedsonia-like organism rather than a virus. Development of symptoms was supressed by chlortetracycline, tetracycline, or chloramphenicol, but not by penicillin. When plants were treated with chlortetracycline at 1000 parts per million before symptoms appeared, symptoms developed only after cessation of the treatment. Assay of the agent of aster yellows, extracted from plants, indicated inhibition of growth of the pathogen by treatment with chlortetracycline. Plants severely affected before treatment began developed new symptomless axillary growth, including flowers; previously yellowed leaves often became green. Acquisition of the agent of aster yellows by leafhoppers was drastically reduced when infected plants were treated with chlortetracycline continuously for 1 week before exposure to the vectors. Our data, and preliminary evidence from purification studies, are consistent with a possible mycoplasma-or bedsonia-like etiology of the aster yellows disease.