Abstract: This work reports the first recorded presence of Mycetophylax simplex (Emery, 1888) in Rio de Janeiro state in Brazil. It also describes the discovery of co-occurrence of the two species of the genus that were previously considered allopatric species. One colony was collected at Praia das Dunas beach in Cabo Frio, Rio de Janeiro state. The co-occurrence of the three species of the genus Mycetophylax indicates that the species are sympatric. The known distribution of the genus Mycetophylax is summarized and discussed.