Objective　To investigate the species composition and seasonality of adult mosquito populations in Baoshan district of Shanghai, China, and to provide reference data for the prevention and control of mosquitoes and mosquito-borne diseases. Methods　Labor hour method was used to determine mosquito population density. Surveillance data collected at the national vector monitoring site in Baoshan district of Shanghai during 2009-2012 were analyzed. Results　Totally 5041 mosquitoes of 5 species were trapped during the four-year study period. Culex pipiens pallens was the predominant species of mosquitos in urban and rural residential areas (94.6% and 98.8%, respectively), while C. pipiens pallens and C. tritaeniorhynchus accounted for large proportions of mosquitos in the barn (46.6% and 23.5%, respectively). Average mosquito population density was lowest in the urban residential area, medium in the rural residential area, and highest in the barn (1.81 vs. 6.70 vs. 30.08 counts per labor hour). There were similar annual trends in seasonal fluctuations of mosquito populations, with high density generally in June to September and low density in January to April as well as November to December. The average value of mosquito population density index progressively increased over the four-year period. Conclusion　No significant changes occurred in the species composition and seasonal fluctuations of adult mosquito populations in Baoshan district of Shanghai during 2009-2012. C. pipiens pallens was the predominant species of mosquitoes in urban and rural residential areas, which, together with C. tritaeniorhynchus, dominated the mosquito populations in the barn. It is recommended to carry out mosquito control from May to October in Baoshan district of Shanghai.