Mafic xenoliths from S-type granitoids in the Lachlan Fold Belt (LFB), Australia, have radioactive heat productions which may exceed those of the host pluton. Mafic xenoliths from the LFB I-type granitoids contain low to moderate radioactive heat productions compared to the host plutons. These LFB granitoid mafic xenoliths indicate that large variations in radioactive heat production could occur beneath the plutons in this area. In the Sierra Nevada batholith (SNB), California, two I-type granitoid groups occur. These are, granitoids having inherently low radioactive heat productions in the western SNB and granitoids having inherently high radioactive heat productions in the eastern SNB. Mafic xenoliths from the eastern SNB granitoids contain radioactive heat productions that in some specimens exceed that found in western SNB granitoids. Therefore, the low radioactive heat productions of the western SNB granitoids can not be used to constrain heat production deep beneath the eastern SNB. ¿ American Geophysical Union 1987