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The density and distribution of Ixodes pacificus was assessed at 2 parks in north coastal California. The density of I. pacificus adults and nymphs varied significantly between years, trails, and sides of trails. Adult ticks occurred on vegetation along sun-exposed trails in January through March, their density (0-1.93 per 20 m) correlated with brush density, trail width, and presence of an uphill slope. Nymphs (0.06-5.10 per 20 m) occurred in leaf litter along shaded trails in May-July. Adult I. pacificus were rare at picnic sites (0.00-0.24 per 20 m), but nymphal densities (0.93-2.37 per 20 m) were comparable with those along some shaded trails. The prevalence of Borrelia burgdorferi in ticks (2.8% overall) did not differ significantly between locations, years, or stages. We conclude that the risk of acquiring
Lyme disease in these sites is low, but varies among trails, seasons, and years.