A dengue fever/dengue hemorrhagic fever (DF/DHF) outbreak in Yap State caused by dengue-4 virus was confirmed serologically and by virus isolation from serum samples collected on each of three island groups. Most DF/DHF cases occurred during a three-month period between mid-May and early August 1995. Five fatal cases, three of which were in children between the ages of four and 11, occurred between June 20 and July 26. A serosurvey conducted in late August revealed anti-dengue IgM prevalence rates of 18% on Yap, 36% on Eauripik, and 6% on Woleai. The majority of residents (93-100%) on the three islands were positive for anti-dengue IgG antibodies, indicating widespread exposure to dengue viruses. The IgG titers indicative of secondary antibody response were noted on Eauripik (6.5%) and Woleai (17%), but were rare on Yap (0.7%). Entomologic investigations implicated the native mosquito species, Aedes hensilli, a member of the Scutellaris Group of Aedes (Stegomyia), as a previously unrecognized epidemic vector of dengue viruses. Aedes hensilli was the most abundant and widespread member of Ae. (Stegomyia) in Yap State, the only species of Ae. (Stegomyia) on Woleai, and the only mosquito species present on Eauripik. New distribution records for mosquito species are reported.