Vulvar phenotypes and the rate of development of eggs over a range of temperatures were used to compare Haemonchus contortus populations in New York State and Ohio. These parameters indicated that the westernmost boundary of the subspecies Haemonchus contortus cayugensis is the Chautauqua valley in New York. The Haemonchus ecotype in Ohio had a vulvar phenotype formula similar to that described for the subspecies Haemonchus contortus contortus. The relationship between slope and intercept of the regression of rate of egg hatch on temperature was different for morphs within ecotypes as well as between the New York and Ohio ecotypes. Linguiform-A appeared to be the cold-adapted morph in both ecotypes. Smooth was the warm-adapted morph in New York with linguiform-B filling that niche in Ohio.
