Carbon dioxide (CO₂) has been used for decades to enhance capture of host-seeking mosquitoes when released in association with traps commonly used by mosquito and vector control agencies. However, there is little published work evaluating the effect of altering CO₂ release rates relative to the number of mosquitoes captured in these traps. This study investigated how varying CO₂ concentration altered the mosquito collection rate at a freshwater wetlands in southern California. Host-seeking mosquitoes were captured in CDC-style traps baited with one of six CO₂ release rates ranging from 0–1,495 ml/min from gas cylinders. Species captured were Aedes vexans, Anopheles franciscanus, An. hermsi, Culex erythrothorax, and Cx. tarsalis. A biting midge, Culicoides sonorensis, was also captured. For all species, increasing CO₂ release rates resulted in increasing numbers of individual females captured, with the relative magnitude of this increase associated to some extent with known feeding preferences of these species. We found that variation in CO₂ release rate can significantly alter mosquito capture rates, potentially leading to imprecise estimates of vector activity if the relationship of CO₂ release rate to mosquito capture rate is not considered.