A survey of the epiphytic flora associated with mangrove roots was undertaken in Málaga and Buenaventura bays, Colombia, in the tropical eastern Pacific. Roots were collected and preserved in 96% ethanol. In the laboratory, all the epiphytic flora was removed and identified. Three females of Anaphes sp. (Hymenoptera, Mymaridae) were found inside thalli of the red alga Catenella caespitosa (Withering) LM Irvine. The three specimens were fully developed but slightly teneral and still in their host eggs, so they could not be identified to species. This is the first record of any insect developing inside the thallus of a marine alga.