Background: Mosquitoes are responsible for spreading devastating parasites and pathogens causing diseases, such as malaria, yellow fever, dengue, Japane encephalitis, filariasis and, more recently Zika virus. The Cyprididan crustasea are the most diverse group of freshwater ostracods.Objectives: The aim of this study was to investigations on natural antagonists of mosquito larvae under field conditions.Materials and Methods: This investigation was carried out from March 2018 until late September 2019 after selecting fixed sampling sites including: Mehr Abad Airport (35.68° N, 51.31° E) and Iran Air Airline (35.69° N, 51.31° E) in Tehran capital town. The morphological characteristics of mosquitos larvae and their associated crustacean were investigated using live observations at Dengue vectors Laboratory, Medical Entomology and vector control, School of Public Health, Tehran University of Medical Sciences.Results A total of 2620 mosquito larvae were collected and mosquitoes under two genera were identified. The recorded species were Culex pipiens Linnaeus, Cx. theileri Theobald and Culiseta longiareolata Macquart, Culex pipiens with 82% was the most abundant species.We found negative interactions between larvae of Cx.pipiens and predatory Cyprididae (Crustacean: Ostracoda), which both were dominant during the summer. Larvae of Cx. pipiens were spatially associated with competing Cyprididae, but they colonized ponds more rapidly. Populations of Cyprididae established in August and prevented Cx. pipiens colonization after that. Ostracoda were highly abundant during the August, but their presence was restricted to decrease of water temperature. The initial development of crustaceans was found in August, when mosquito larval colonization was strongly inhibited. This study showed that micro-crustaceans are relevant for mosquito management but their impact on mosquito larvae varies between species and depends on environmental conditions.Conclusion: This investigation provides insights for mosquito larvicidal action of aquatic Cyprididae on different larval stages of Cx.pipiens. It could be suggested as a potential candidate in mosquito biocontrol programs.