Following the discovery of the introduced mosquito, Ochlerotatus japonicus in western New Hampshire in 2000, a series of twelve used tire casings in southeastern New Hampshire was sampled, beginning in 2001, to study the occurrence of O. japonicus and its abundance over a five year period. Prior to 2003, Ochlerotatus triseriatus, a native species, was the numerically dominant mosquito species found in older water-filled tire casings. By 2004, O. japonicus had replaced O. triseriatus as the dominant mosquito species in this habitat, and by 2006 comprised 83% of the 2 species collected from the tires. Ochlerotatus japonicus has two larval population peaks annually in southeastern New Hampshire during the breeding season: mid-April to mid-May in spring, and early August to mid-September in mid-to late summer. Eggs continue to hatch and some larvae are present until freezing (ice over) occurs, usually in late October to early November. Ochlerotatus triseriatus has a single larval population peak from early May to mid-June. Eggs cease to hatch by late July to mid-August.