The brown citrus aphid, Toxoptera citricida (Kirkaldy), is distributed widely on citrus but is not known to occur in the Mediterranean region or, until recently, in North America. The brown citrus aphid is an important pest of citrus because it is an efficient vector of citrus tristeza virus. Surveys have shown that the brown citrus aphid has rapidly expanded its distribution in the Caribbean Basin and Central America. In September 1991, the brown citrus aphid was found widespread on citrus in Costa Rica, Nicaragua, and Panama but was not found in Belize, El Salvador, Guatemala, Honduras, or Mexico. In 1992, it was found in the Dominican Republic, Guadeloupe, Haiti, Martinique, Puerto Rico, St. Croix, St. Thomas, Trinidad, and the U.S. Virgin Islands and, in 1993, in Cuba and Jamaica. The aphid was not found in the Bahamas or Bermuda in the survey. Enzymelinked immunosorbent assay tests from citrus trees indicated that citrus tristeza virus incidence was low (<15%) in most areas and probably was caused by the use of infected budwood, rather than by aphid vectors, because of its random pattern of a few infected fields separated by tristeza-free fields. Most citrus tristeza virus isolates detected did not react with the severe-strain discriminating monoclonal citrus tristeza virus antibody, MCA13, and, thus, are probably mild strains. Some MCA13 reactive isolates were found in Belize, Costa Rica, Panama, Bermuda, the Dominican Republic, Jamaica, Puerto Rico, and Trinidad. The brown citrus aphid was ≍6-25 times more efficient in transmitting several strains of citrus tristeza virus than the melon aphid, Aphis gossypii Glover. Incidence of decline isolates of citrus tristeza virus could increase rapidly with the advancing movement of the brown citrus aphid and could threaten ≍180 million citrus trees on citrus tristeza virus-sensitive sour orange rootstock in the Caribbean Basin, Central America, Mexico, and the United States. An additional threat exists if this aphid begins to spread grapefruit and sweet orange stem-pitting strains of citrus tristeza virus.