Coprologic examination of nine Siberian chipmunks (Eutamias sibiricus) imported from Southeast Asia revealed infection with Cryptosporidium sp. Experimental inoculation of BALB/c mice proved their susceptibility to the infection. Infected mice shed oocysts 14-35 days postinfection. Oocyst morphology was similar to that reported for C. muris in previous studies, oocysts were 8.1 (7.0-9.0) x 5.9 (5.0-6.5) microns. Clinical signs were absent in naturally infected chipmunks and experimental mice. Histologic examinations of mice revealed numerous developmental stages of C. muris in the glandular stomach. Analysis of partial small subunit rRNA gene sequences confirmed identity of these isolates as C. muris. Our results represent the first report of C. muris in members of the family Sciuridae.