The Republic of Korea (ROK) has been experiencing an expanding epidemic of Plasmodium vivax malaria since 1993, with most cases occurring near the Demilitarized Zone in the northwestern part of the country. During 1996 and 1997, U.S. Army preventive medicine assets undertook a program of surveillance and vector control to reduce transmission to U.S. Forces Korea (USFK). In 1996, the total number of cases was low and only routine vector control (ULV applications of malathion in early evening, availability of repellent for soldiers without command emphasis) was undertaken. Surveillance in 1996 indicated that Anopheles sinensis Wiedemann was the vector and that the risk to unprotected troops was high (1 of 361 mosquitoes infected in one area with 30 bites/person per night, 1 of 1,559 mosquitoes infected in another area with biting rate of 130/person/night). All night collections showed that most biting was occurring late at night, between 2300 and 0300 hrs. Expecting more transmission in 1997, a coordinated program (which did not include chemoprophylaxis) was initiated in which physicians, military commanders, and entomology assets all played a role. The following entomological interventions were concentrated at the site of highest risk, Camp Bonifas: 1) application of residual insecticide to tents, barracks, and bed nets, 2) ULV application of resmethrin late at night, 3) replacement and repair of screens, 4) use of permethrin treated bednets in unscreened billets, and 5) command emphasis on use of repellents and proper wear of the permethrin-treated uniform. Camp Bonifas experienced a 40% decrease in malaria cases (from 5 cases in 1996 to 3 cases in 1997) compared to a 243% increase in other American troops (from 7 to 24 cases) and a 306% increase (from 285 to 1,156 cases) in ROK Army troops. Although more effective use of ULV and application of larvicides would probably have improved the result, we conclude that entomological interventions without chemoprophylaxis reduced malaria transmission at Camp Bonifas by 82% in 1997.