Icosahedral viral particles were found in the cytoplasm of erythrocytes and splenic reticular cells of a marine toad (Bufo marinus) collected from Costa Rica. Capsids had a maximum diameter of 312 nm and a spherical core with biphasic electron density. Viruses in erythrocytes were associated with cytoplasmic assembly areas and vacuoles in cytoplasm. Nuclei had finely granular material of decreased electron density located centrally, but contained no viral particles. A group of unenveloped viral particles was seen extracellularly in a splenic vessel. The virus was consistent with an iridovirus. In a blood smear stained with Giemsa round basophilic bodies with average diameters of 1.70 microns and morphologically similar to Pirhemocyton sp. were seen in the cytoplasm of erythrocytes and occasionally in the cytoplasm of monocytes or extracellularly. Erythrocytes containing these bodies had vacuoles and irregular pale-staining areas in the cytoplasm and pale-staining areas in the nucleus. These changes corresponded to the viral particles, assembly areas, vacuoles and nuclear changes at the ultrastructural level.