The tracer diffusion of64Cu in lithium has been measured. The results are expressed byDCu&sngbnd;Li=(0.47±0.11) exp[(−9.22±0.22)/RT] (cm2/sec).Copper diffuses faster than any other hitherto investigated impurity in lithium; close below the melting pointDis about 2×10−6cm2/sec. From the magnitude of the diffusivity, from the systematics among the noble metals, and from parallels with diffusion results in certain polyvalent matrices, it is inferred that Cu, Au, and Ag impurities in Li diffuse as interstitials.