Ochlerotatus japonicus japonicus is reported for the 1st time south of the Mason-Dixon Line, in Frederick County, Maryland. Fifty-seven oviposition trap samples were collected throughout the county between June 30 and August 24, 2000. From 971 larvae reared from the oviposition traps, 5 species were identified: Ochlerotatus triseriatus (45%), Oc. j. japonicus (43%), Aedes albopictus (7%), Culex pipiens (4%), and Toxorynchites ritulus septentrionalis (<1%). Ochlerotatus j. japonicus was found widely distributed over the area sampled. This is the 1st record of Ae. albopictus in the county as well. Vector competence studies indicated that Oc. j. japonicus is an efficient laboratory vector of West Nile (WN) virus. Depending on the viral titer at time of feeding, the estimated transmission rates for Oc. j. japonicus for WN virus were 2-4 times higher than that for Cx. pipiens. Studies of the viral titer in mosquitoes over time showed that titers in the bodies of infected Oc. j. japonicus reached their peak (approximately 10(6.5) plaque-forming units/mosquito) between 7 and 11 days after taking an infectious blood meal, and that virus became detectable in the legs (an indicator of disseminated infection) as early as 3 days after taking an infectious blood meal.