Technical Abstract: TECHNICAL ABSTRACT: Aphis gossypii is the primary vector of Citrus tristeza virus (CTV) in California and exists as two distinct biotypes: the melon aphid; and the cotton aphid. These biotypes are morphologically indistinguishable but have distinct host ranges. Both aphids have a wide host range outside of citrus and presumably feed and colonize citrus during the aphid's peak migration periods. Both A. gossypii biotypes were established in the laboratory and the host range, feeding behavior, and propensity for the transmission of CTV were examined. The melon aphid fed and developed on other hosts more than the cotton aphid. Preliminary analysis of probing behavior examined by direct current (DC) electrical penetration graph (EPG) monitoring on seedlings of the citrus cultivars Madam Vinous and Mexican lime indicated that the melon aphid probed more and ingested longer on both hosts than the cotton aphid. The melon aphid transmitted three of four CTV isolates with higher efficiency than the cotton aphid using Mexican lime as receptor plants. The role of the two biotypes of A. gossypii in relation to vector efficiency and epidemiology of CTV will be discussed.