Filarial parasites are a type of nematode that requires arthropod vectors for transmission between hosts. Filarial parasites vary among species of vertebrate hosts and can cause varying symptoms in hosts, including death. The presence of filarial parasites can influence host populations and can be costly to infected areas. To evaluate the prevalence of filarial parasites in Lake County, CA, mosquitoes were collected in 2014 and analyzed for infection using polymerase chain reaction (PCR). Of 1,008 mosquito pools, six filarial parasite species were detected in 23 pools representing six mosquito species. The DNA of Dirofilaria immitis (n=6, MIR=0.26); Setaria yehi (n=9, MIR=1.44); Splendidofilaria sp. (n=4, MIR=0.20); unknown filarial parasites A (n=2, MIR=0.09), B (n=1, MIR=0.41), and C (n=1, MIR=0.05) were detected in Aedes increpitus, Aedes sierrensis, Anopheles franciscanus, Anopheles freeborni, Culex stigmatosoma, and Culex tarsalis mosquito pools. Due to the presence of D. immitis, which can lead to chronic illness and death in domestic dogs, in Lake County it is important to evaluate vector competency of D. immitis in mosquito species. Culex tarsalis was chosen due to the high abundance found in Lake County in 2014 (n=36,587). To evaluate vector competency of Cx. tarsalis in transmission of D. immitis, colony and field-caught Lake County (n=102, n=54 respectively) mosquitoes were analyzed for infectivity using decapitation. Fourteen days post feeding on infected blood, mosquitoes were decapitated to evaluate the presence of L 3 larvae; but no L 3 larvae were detected. The presence of D. immitis DNA was detected in eight colony (IR=7.8%) and fifteen field-caught (IR=23.1%) thoraces using PCR. Though no L 3 larvae were observed in decapitated mosquitoes, presence of D. immitis DNA in the thoraces of mosquitoes using PCR has previously been used as an indicator for vector competency. Thus it is probable that Cx. tarsalis is a competent vector for D. immitis.