Abstract.The mosquitoesAedes notoscriptusandCulex annulirostris, previously shown to be competent vectors ofDirofilaria immitis, were allowed to feed on dogs with densities of microfilariae ranging from 2665 to 33,055 microfilariae/ml. Filaria developed in both species at a similar rate independent of the initial microfilarial density.Cx annulirostrisingested more microfilariae although the number of developing larvae was consistently greater inAe. notoscriptus;one femaleAe. notoscriptuscarried sixty‐two third‐stage larvae 10 days after feeding, although no more than twelve were found inCx annulirostris.The mortality rate ofAe. notoscriptusin the first 3 days of filarial development was related to increasing microfilarial density, and was greater than that ofCx annulirostris, which quickly reduced its worm load and survived with fewer third‐stage larvae but for longer thanAe. notoscriptus.The vector efficiency index was higher at all microfilarial densities forAe. notoscriptus, but its survival was much reduced, and thereforeCx annulirostrismay be the more effective vector in localities where dogs have very high parasita