Lethal yellowing (LY) di9ease developed in three of five coconut palms, Cocos nucifera L. five of seven Manila palms, Veitchia merrillii (Becc.) H.E. Moore, and two of three of the palm, Pritchardia thurstonii F. Muell. et Drude, that were planted in cages into which ? 850 wild planthoppers, Myndus crudus Van Duzee, were introduced each month for 34 months.. Transmission of LY disease to one or more palms occurred in each of five treatment cages. Mycoplasma-like organisms (MLO) were observed in sieve elements in tissues sampled from all palms that developed LY symptoms, but not in samples from healthy palms. Palms of the same species in five cages in which M. crudus were not introduced remained healthy. Insects that contaminated the experiment were discounted as possible vectors by biological and experimental design considerations. The number of M. crudus used in the experiment and the susceptibilities of the palms were consistent with field observations. The results of this experiment provide the strongest evidence yet published that M. crudus is a vector of LY agent.