This article explores one of the earliest large-scale uses of biocidal agro-chemicals in Latin America, the United Fruit Company's hand-spraying of its banana plantations to control sigitoka disease from 1938 to 1962. After discussing the environmental context of sigatoka and the early development and implications of the spray technology, the essay focuses on the thousands of workers who applied the chemicals. Using Costa Rica as a case study, it explores workers' sense of the personal costs of their work as well as their ambiguous relationship to the larger banana workers' union movement. Because of differences in ethnicity, age, and masculine status, pesticide workers were not part of the labor movement's militant core, but their participation in strikes gave unions great power for a time. This power, along with workers' individual job actions, forced fundamental changes in the pesticide program, demonstrating the importance of integrating labor into the study of environmental change in agricultural capitalism.