Seven hundred and twenty five female culicid mosquitoes were captured in an area of Western Spain where Dirofilaria immitis is endemic. PCR was applied for the detection of larval D. immitis DNA in mosquitoes. The ITS-2 rDNA region of the mosquito DNA was also amplified and sequenced to characterize the haplotype of infected individuals. Two Cx. pipiens showed positive reaction to D. immitis DNA, both specimens belonging to the haplotype H1. It is the first time that a culicid mosquito species is incriminated in the transmission of D. immitis and that its haplotype identified in Spain. The anthropophilic behaviour of this species, and its capture with human bait as attractant, led the authors to speculate that Cx. pipiens could be implicated in the transmission of human dirofilariosis in this endemic area.