Fifty-four black-tailed jackrabbits (Lepus californicus) (five juvenile males, 22 adult males, five juvenile females, and 22 adult females) from Humboldt County, California (USA) were evaluated for sex and age-specific differences in parasite prevalences and intensities, 26 February through 30 October 1996. Nematodes found included Biogastranema leporis in 42 hares (78% prevalence), Rauschia triangularis in 26 hares (48%), Trichostrongylus calcaratus in 14 hares (26%), and Trichuris sylvilagi in two hares (4%). Cestodes found included Taenia sp. cysticerci in five hares (9%) and Taenia sp. coenurus found in one hare (2%). Ectoparasites found included the ticks Dermacentor variabilis on 10 hares (19%) and Ixodes spinipalpis (=Ixodes neotomae) on nine hares (17%), as well as the anoplurid louse Haemodipsus setoni on 12 hares (22%). No significant differences in the parasite prevalences or intensities were found between male and female jackrabbits; this was for all males and females collectively, juvenile males and females only, as well as adult males and females only. Combining male and female hosts, adult jack-rabbits had a significantly higher prevalence of B. leporis and R. triangularis compared to juveniles. This is the first known report of Trichostrongylus calcaratus, Rauschia triangularis, Trichuris sylvilagi, and Dermacentor variabilis among black-tailed jackrabbits and the first known report of T. calcaratus and T. sylvilagi in the western USA. This is the first published report of I. spinipalpis, the vector for Lyme disease in California, on black-tailed jackrabbits.