One new species of the genus Coreomyces was collected on Micronecta sedula from the island of Java; C. javanicus sp. nov. is very similar to and apparently closely related to C. corixae. This species is characterized by a receptacle that is bent towards one side with cell I, nearly rounded. The length of cell I is equal to that of cell II and the perithecia are longitudinal, elliptical or cylindrical. Thalli consistently occurred on the margin of the left elytron of the hosts.