We have measured the magnetoresistance for the heavy-fermion system U1-xThxBe13 for x=0, 0.006, 0.015, and 0.033 at temperatures between 1.2 K and 6K. The samples with x=0.006 and 0.015 develop a positive magnetoresistance as the temperature is increased above approximately 2 K. We observe the large negative magnetoresistance for the pure UBe13 and the sample with x=0.033, in agreement with the results of previous studies. © 1990.