This report describes a case of peritoneal larval cestodiasis caused by tetrathyridia of IVIesocestoides sp. in an adult female squirrel monkey. The monkey had lived in a zoological garden in Japan and had a clinical history of wasting. At necropsy, numerous whitish oval masses were found in the liver and peritoneal cavity. These masses contained larval cestodes. Morphological observation and molecular analyses of the mitochondrial 125 ribosomal RNA gene and cytochrome c oxidase subunit 1 gene sequences allowed us to identify the larva as the tetrathyridium of Mesocestoides sp. This is the first report of Mesocestoides larvae in a squirrel monkey in Japan. (C) 2014 Elsevier Ireland Ltd. All rights reserved.