An adult female American alligator (Alligator mississippiensis) had diffuse, yellow, granular serosal thickening at necropsy. Light microscopic examination of affected stomach, small intestine and spleen revealed a chronic proliferative serositis associated with 3 to 15 μm eosinophilic extracellular globules identified histochemically and morphologically as egg yolk. The intracoelomic egg yolk was considered to be the cause of the serosal reaction.