This report describes a reactive mesothelial hyperplasia as a reaction to a polyserositis in an African green monkey (Chlorocebus aethiops) mimicking an epitheloid mesothelioma. Histopathology, immunohistochemistry, fluorescence in situ hybridization, and electron microscopy were used to determine the benign nature of the mesothelial proliferation. For the first time, human genetic probes had been successfully applied to an African green monkey.