Haemonchus contortus is a blood-sucking abomasal helminth of small ruminants responsible for major economic losses to producers worldwide. Widespread resistance to commercial anthelmintics has created a need for alternative methods of parasite control. One method involves using plant products with natural anthelmintic properties. This thesis assessed the efficacy of several plant products against experimental Haemonchus contortus infection in gerbils and sheep. In gerbil assays, animals were orally infected with 600 third-stage Haemonchus larvae and treated once or daily for 5 days with artemisinin, Artemisia annua aqueous or ethanolic extract, an orange oil emulsion, or Asimina triloba ethanolic extract. Nine days post-infection, gerbils were euthanized, their stomachs removed, and the worms counted. Significant anthelmintic activity was not found for artemisinin, A. annua extracts, or A. triloba extract. The orange oil product caused significant parasite reductions up to 87.8% when administered for 5 days. The orange oil emulsion was tested in sheep to evaluate the product against Haemonchus in its natural host. Sheep were orally inoculated with 10,000 Haemonchus larvae and, one month later, dosed with the emulsion once or daily for 3 days. Fecal egg counts were monitored daily starting on the first day of dosing and continuing to 14 days post-dosing. Results showed that a single dose of the product caused highly significant fecal egg count reduction (97.4%) compared to control sheep and that there is no advantage to treating for 3 days. Thus, the orange oil emulsion shows promise as an alternative to commercial dewormers. / Master of Science