Experiments were conducted to compare the relative pathogenicity and infectivity of deer- and cattle-derived Haemonchus contortus for three hosts, viz., white-tailed deer, cattle and domestic sheep. Parameters evaluated for all animals were: general physical condition, basic hematologic values, fecal egg counts and parasite infectivity rates. Clinical signs attributable to H. contortus infections were not observed in any of the experimental animals. Deer harboring H. contortus burdens greater than 70 worms/kg body weight had decreased packed cell volume, hemoglobin and total serum protein values. Statistical analyses indicated there was not a significant difference (P greater than .05) in infectivity of deer-derived H. contortus in these hosts. No significant difference (P greater than .05) in infectivity for deer was noted between deer-derived H. contortus and cattle-derived H. contortus. Morphometric comparisons of helminths recovered indicated that parasites of deer and cattle origin were both compatible with the description for H. contortus. Results suggest that cross-transmission of H. contortus occurs between deer and domestic livestock.