An 8-year survey of filarial worm infections in black-tailed deer (Odocoileus hemionus columbianus) in a northern California study area revealed that the great majority of deer became infected with three filariids. The footworm (Wehrdik-mansia cervipedis) and the arterial worm (Elaeophora schneideri) showed increased prevalence with host age but just the opposite was seen with the abdominal worm (Setaria yehi). Most fawns were infected with S. yehi as were almost half of the yearlings but the parasite was relatively scarce in deer over 2 years of age.