A new minute, slender demodicid, Demodex leucogasteri, is described in all stages of the life cycle from Onychomys leucogaster, the grasshopper mouse. All stages are located in the hair follicles: adult mite at or above sebaceous gland ducts, immatures and ova at or below these ducts . . Mites are generally distributed in all body areas but are most common in muzzle and eyelid. The sex ratio (1: 5.8) indicates arrhenotoky. This species is apparently a typical low grade pathogen - merely harvesting individual cells of the follicular epithelium, causing minor distension and hyperplasia of the follicle and occasional hair loss.