This USGS data release consists of six data sets and accompanying metadata for a year-long study on the seasonal distribution and relative abundance of the southern house mosquito Culex quinquefasciatus in Kīpahulu Valley, Haleakalā National Park, critical habitat for two endangered Maui endemic forest birds. Culex quinquefasciatus is the vector of the avian malaria Plasmodium relictum, a key limiting factor of endemic Hawaiian forest birds. The main components of the study included: (1) adult mosquito monitoring to document the altitudinal distribution, seasonal occurrence, relative abundance, and trapping efficacy of Culex quinquefasciatus (Kīpahulu Valley, Maui, 1 file); (2) stream surveys to determine the larval habitat availability and prevalence of larval mosquitoes (Kīpahulu Valley, Maui, 1 file), (3) characterization of larval habitat (Kīpahulu Valley, Maui, 1 file), (4) egg viability (hatching rate) in naturally occurring Culex quinquefasciatus populations (Kīpahulu Valley, Maui and Volcano, Island of Hawai'i, 2 files) and (5) the geographical coordinates for key localities in the study (Kīpahulu Valley, Maui and Volcano, Island of Hawai'i, 1 file). The study was conducted by U.S. Geological Survey, Pacific Island Ecosystems Research Center personnel in collaboration with National Park Service, Pacific Island Inventory and Monitoring Network, Haleakalā National Park and Hawai‘i Volcanoes National Park. Studies were conducted in native forests on Maui and the Island of Hawai'i during 2022 and 2023.