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Between 18 June and 20 September 1986, 28 cases of Plasmodium vivax malaria were documented in Carlsbad, California, a coastal town north of San Diego. Malaria occurred in 1 local resident who had no risk factors, a second local resident who had traveled to a malarious area 9 months earlier, and 26 Mexican migrant workers (MWs). Among the 28 cases, 27 lived in a square mile marshy area where Anopheles hermsi, a newly described American species of the Anopheles maculipennis group, was known to be breeding. An investigation of MWs residing in the affected area was done to determine the extent of the outbreak and to identify risk factors for acquiring malaria. We interviewed and drew blood from 304 healthy MWs and 17 (65%) of the MWs with malaria. Fluorescent antibody titers to P. vivax ≥1:256 occurred in 14 (82%) of the 17 MWs with malaria tested and 9 (3%) of the healthy MWs. The principal risk factor identified for contracting malaria was sleeping outside on a hillside adjacent to the marshy area. Malaria in a local resident with no malaria risk factors and the clustering in time and place of 26 cases suggest that P. vivax malaria was introduced and local transmission was sustained through several generations, producing the largest outbreak of introduced malaria in the United States since 1952.