Thirty-seven subadult and adult coyotes (Canis latrans), collected August 1992 through December 1996 from a coastal foothill area in northern California (USA), were examined for adult heartworm (Dirofilaria immitis). During 1992 through 1993, at the end of a 6 yr drought, none of four coyotes examined were infected with heartworms. However, during 1994 through 1996, after the drought had ended, prevalences were 91% in 23 adult coyotes and 40% in 10 subadult coyotes. Heartworm intensity did not differ by sex of coyote, and averaged (±SE) 19.4 ± 3.8 among adults; one subadult had >238 heartworms. The prevalence and intensity of heartworm infection in coyotes reported here for 1994 through 1996 are the highest reported anywhere in the United States.