In this study, the antagonistic effect of the bacteria Streptomyces griseoviridis on the fungi Mycena citricolor, which causes the American leaf spot disease on coffee plants, was evaluated. The purpose of experiment was to determine if the bacteria had any antagonistic effect over the growth of the fungi. As well, as part of the study, a field experiment was conducted in a coffee plantation in Turrialba, Costa Rica. In this study three treatments were evaluated; the first treatment using S. griseoviridis, the second using the fungicide cyproconazole, and the third a control treatment. The experiment used a randomized block statistical design with four repetitions. Each repetition consisted of four coffee plants, each having four branches, which were evaluated during the experiment. Three applications were made between May and August, and at the same time four evaluations were made to determine the percentage of infected leaves and the severity of the disease. Cyproconazole was the treatment that had the best at controlling the American leaf spot disease of coffee compared to the other treatments. S. griseoviridis did not show any antagonistic effect at the evaluated dosage.