Nymphal and larval stages of Ixodes (Ixodes) jellisoni Cooley & Kohls and I. (I.) neotomae Cooley are described for the first time. These 2 tick species occur only in the western United States, predominantly in California. The primary host for I. jellisoni is the California kangaroo rat, Dipodomys californicus (Merriam); that for I. neotomae is the dusky-footed woodrat, Neotoma fuscipes Baird. The etiologic agent of Lyme disease Borrelia burgdorferi Johnson, Schmidt, Hyde, Steigerwalt & Brenner has recently been isolated from both tick species, and I. neotomae was proven a competent enzootic vector of the Lyme disease spirochete.