Widespread anthelmintic resistance in trichostrongyle nematodes of ruminants has created an urgent need for alternatives to commercial anthelmintics. The bacterium Bacillus thuringiensis (Bt) can produce crystal proteins during sporulation, which can be lethal to insects in multiple orders when ingested. One protein, Cry5B, has demonstrated effectiveness against multiple parasitic nematodes. We hypothesized that Cry5B would be effective against Haemonchus contortus, a highly pathogenic parasite, in sheep. Two experiments tested efficacy of Cry5B in sheep experimentally infected with H. contortus. In the first, a live genetically modified, asporogenous strain of B. thuringiensis expressing cytosolic Cry5B protein (BaCC) was administered orally daily for four days (~40mg/kg Cry5B/day). The mean fecal egg count (FEC) of treated animals was reduced by 94% three days after treatment, and at necropsy the female worm burden was significantly reduced by 98%. In the second experiment inactivated, asporogenous Bt expressing cytosolic Cry5B (IBaCC) was used. Treated animals received 60mg/kg Cry5B, administered daily for three days. By 72 hours after the first treatment FEC was reduced by 91%. Mean total worm burden of treated sheep at necropsy was significantly reduced, with female worms reduced by 95%. A third study tested the effect of BaCC and IBaCC on development of eggs to infective larvae in feces under laboratory and outdoor environmental conditions. Cry5B (15mg) added to feces (10g) reduced numbers of infective larvae by 99% in both environments within 12 days. Cry5B appears to have potential for controlling H. contortus in sheep. All protocols approved by VT IACUC and IBC. / Master of Science / Many animals and humans can be infected with roundworm, also called nematode, parasites. Infection of animals and humans by parasitic nematodes can result in disease. Some animals like ruminants (cows, sheep, and goats) can be infected with multiple species at once with few effects on the host. However, certain species can cause major disease, and even kill their ruminant host. Younger animals like lambs can easily become overwhelmed by these parasites. Anthelmintics are the type of drug used to treat those infected with these parasitic worms. However, just like bacteria are becoming resistant to antibiotics, these worms are also becoming resistant to anthelmintics. Because of this, researchers are looking for new compounds and materials that are lethal to the parasite and can be used to treat infected animals. One species of bacterium, Bacillus thuringiensis, is usually found in the soil. This bacterium can produce a large crystal structure that is made up of proteins. These crystal (Cry) proteins can be lethal to pest insects like beetles, caterpillars, and mosquitos. When the insect eats the protein, it binds to cells in the insect intestine, creating holes in the insect gut. These proteins can be lethal to nematodes as well when they are eaten by the worms. Because of this, these proteins are being investigated as potential alternative treatments for parasitic nematodes. One type of protein, Cry5B, has been tested in hamsters, mice, and pigs. We hypothesized that Cry5B would also be effective against a sheep stomach worm called Haemonchus contortus. We tested the Cry5B in two different formulations and found that the protein was effective against both the adult worm in the stomach, and the young worms in the feces of the host. This protein could potentially be used to treat parasitic nematodes that have become resistant to anthelmintics.