A retrospective case-control study was conducted to identify risk factors, other than contact with infected animal tissues, for infection caused by Yersinia pestis in non-Indian residents of areas with endemic sylvatic plague in New Mexico. The study group consisted of 16 persons who had bacteriologically confirmed cases of plague in the period 1975–1976 and 45 controls who were matched by age, sex, and location of residence. Participants were questioned about outdoor activities, environmental conditions in and around the home, procedures of pet care, exposure to animals and insects, and medical history. The availability of harborage and food sources for wild rodents as a result of human activity in the immediate home environment was associated with risk of infection. Failure to take steps to control fleas on dogs and cats also appeared to be a possible risk factor. No other significant differences were found between patients and controls. Plague in New Mexico appears to result primarily from entry of the reservoir host into the habitat of the human rather than from entry of the human into the sylvatic habitat of the reservoir host.