In a mosquito survey of the Republic of Costa Rica 14,961 adult mosquitoes and 9,743 larvae were examined, representing 93 different species. Greater attention was paid to the Anopheles than to any other group. Tables were drawn up to show the relative prevalence of the various anophelines and culicines, when caught as adults or when taken in their immature stages. In addition, we have listed the distribution records of all the species found during this survey, together with notes on the types of breeding places preferred by them. Keys for the adult females of many of the Costa Rican mosquitoes have been included, as well as the names of certain other biting insects which we have been able to identify from this country.