Prevalence rates of heartworm in coyotes was estimated for five northern California counties by examining hearts of coyotes obtained from federal or private trappers. Prevalence rates range from 8.3-27.3%, but the proportion of infected coyotes can vary between two regions of the same county. In general, it seems that higher infection rates are found in the foothill regions of the Coastal Range and Sierra Nevada, decreasing towards the floor of the Sacramento Valley as well as at higher elevations. The use of coyotes as sentinel animals for detecting natural foci of transmission is postulated.