An extensive epidemic of tularemia with 529 cases, 400 of which were confirmed by laboratory tests, occurred in the northern part of central Sweden during the summer of 1981. The outbreak was of short duration and was restricted to certain communities within a narrow geographical area. It began in the middle of July and progressed during that month and August, with only sporadic cases in September and October. During the 2 years preceding the outbreak only 3 and 7 cases were reported in Sweden. The infection was mainly transmitted by mosquitoes and most cases were ulceroglandular. The later cases in September and October were infected by contact with hares or rodents. All age groups were affected, with a slight predominance of women and the 30-60 yr age groups.