Seasonal variations in the density of questing Ixodes ricinus (L.) nymphs and their prevalence of infection with Borrelia burgdorferi s.l. Johnson, Schmid, Hyde, Steigerwalt & Brenner were studied during 1991-1993 at a locality in southcentral Sweden. The seasonal pattern of the density of questing I. ricinus nymphs was variable; there was a bimodal pattern with peaks of similar magnitude in 1991, a bimodal pattern with a small spring peak and a greater autumn peak in 1992, and a unimodal pattern with a spring peak and lower values during summer and autumn in 1993. The seasonal pattern of prevalence of Borrelia infection in questing nymphs was similar during all 3 yr, with a peak infection prevalence in late spring or early summer, followed by lower infection prevalences in late summer and autumn. There was a significant correlation between the monthly density of Borrelia-infected nymphs and the monthly density of nymphs. This suggests that, within a certain range of nymphal densities, it may be possible to assess the density of Borrelia-infected I. ricinus nymphs without examining nymphs for B. burgdorferi s.l.