A skin test survey with paracoccidioidin and histoplasmin was conducted on 495 residents of Coari in the Brazilian state of Amazonas. Positive dermal reactions to paracoccidioidin and histoplasmin were seen, respectively, in 13.9% and 50.1% of the study subjects. Specific reaction to paracoccidioidin was observed in 1.6% of the individuals, and to histoplasmin in 37.8%. Cross-reactivity between the two antigens in persons who reacted positively to both skin tests was not totally reciprocal. There were no significant differences in dermal reactivity to either of the two antigens in either sex, or among different occupational groups. No precipitating antibody to paracoccidioidin or histoplasmin was detected in the sera of the skin test-positive individuals by the immunodiffusion test. Skin test positivity in the general population of Coari reaffirmed the concept of benign, self-limited infection in paracoccidioidomycosis and histoplasmosis. It also identified the equatorial forests of the Upper Amazon Basin as endemic zones of these two mycoses.