In 1997, Aedes albopictus (Skuse) was discovered in Peoria, IL, a known focus of La Crosse (LAC) virus transmission. This accidental introduction provided an opportunity to determine whether Ae. albopictus would reemerge in the spring or summer and, if successful overwintering occurred, to follow changes in the geographic range of Ae. albopictus, and to compare its distribution to that of the local treehole mosquito and LAC vector Aedes triseriatus (Say). In 1998, 25 oviposition traps were placed in and around the area of the initial finding of Ae. albopictus, with adult collections by aspirators and larval collections from water-holding containers used to identify areas of additional activity. Ae. albopictus successfully survived the mild 1997-1998 El Niño winter, and expanded its range during 1998. By September 1998, Ae. albopictus oviposited in all 25 traps, including traps near and in wooded sites. Intensity of oviposition activity (number of eggs per positive trap) ranged from 20-40, lower than the range for Ae. triseriatus, which was as high as 175 eggs per infested trap in mid-August. Prevalence of Ae. albopictus increased through September, where as the prevalence of Ae. triseriatus declined starting in mid-July. Although direct competition between the two mosquito species cannot be inferred based on this surveillance effort, a gradual range expansion was observed, and Ae. albopictus eggs were collected in traps where initially only Ae. triseriatus oviposition activity was detected.