Both Aedes sierrensis and Dirofilaria immitis have recently become established in Utah. We evaluated the vector potential of this Aedes sierrensis strain using a new technique for detecting Dirofilaria immitis in individual mosquitoes. Survival of Aedes sierrensis females after bloodfeeding did not differ from that of Ae. triseriatus but infective Ae. sierrensis produced significantly more L3 nematodes. This observation and epidemiological data support the hypothesis that Ae. sierrensis is the vector of canine heartworm in Utah. Infectivity was determined by counting infective-stage parasites that migrated into the medium after individual mosquitoes were decapitated or crushed in the wells of tissue culture plates. Complete recovery of infective-stage nematodes was attained in 60-74% of the mosquitoes and 77-93% of all L3 were collected with this technique. There were few false negatives. High recovery rates (mean = 89%) were also obtained for mosquitoes treated en masse.