Juveniles identified as D. immitis were recovered from A. trivittatus and Anopheles punctipennis collected with CO2-baited CDC [Center for Disease Control] light traps in Ames, Iowa, USA, during 10 days in July and Aug. 1975. A. trivittatus composed 58.7% of the mosquitoes collected, and D. immitis was recovered from 18 of 393 (4.6%) dissected and from 7 of 31 pools (911 mosquitoes; 0.77%). About 1.0% of the A. tribittatus collected harbored infective-stage juveniles, and as many as 27 were recovered from a single mosquito. This is the 1st report of A. trivittatus supporting the development of D. immitis. A. punctipennis was the only other species infected, with 2 of 468 harboring 1st-stage juveniles. A. trivittatus probably is the principal vector of D. immitis in central Iowa.