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The in vitro lethal activity of two Bacillus thuringiensis strains (IB-16 and IB-61) against Haemonchus contortus histotropic fourth stage larvae (L(4)) was evaluated. Each B. thuringiensis strain was used as soluble proteins at different concentrations. In addition, the in vitro lethal dose (LD(50)) of each strain was determined against H. contortus L(4) at different times. Data were analyzed by ANOVA and Tukey tests. Less than 50% mortality was observed at 50 and 100 microg/mL during all bioassays. More than 50% mortality was observed at 200 microg/mL at 3 and 5 days after interaction with IB-16 and IB-61, respectively. A 100% mortality was observed with IB-16 on day 5. The LD(50) with IB-16 was 248 microg/mL on day 3, and IB-61 showed an LD(50) at 227 microg/mL on the day 5. Both toxins may have potential as biocontrol agents.