Aedes albopictus and Ochlerotatus j. japonicus are mosquitoes that have been recently introduced into the United States. Since their introduction, they have been implicated in the transmission of one or more of the arboviruses of principal public health importance in the mid-Atlantic region. To more fully understand the potential of Ae.albopictus and Oc. j. japonicus to be vectors of endemic arboviruses, field and laboratory studies were conducted to assess their distribution in the region and to determine their vector competence for arboviruses for which data are lacking. Aedes albopictus, a mosquito found in virtually all counties of states in the southeastern United States, was found to be established as far north as south-central Pennsylvania. Ochlerotatus j. japonicus, a mosquito that is generally found in more northern climates within its native range, was found to be established as far south as Maryland, and its relative abundance in Frederick County, Maryland, was found to be comparable to or greater than that of other container-inhabiting mosquito species.