The radioactive isotope, 65Zn was fed and injected into rabbits, opossums, foxes, and bobcats. When injected, 65Zn was detectable in feces from these animals for over a year after injection. The radioactive zinc present in the fecal pellets is also very stable to weathering. The injection of 65Zn is an excellent technique for long-term marking of feces in censusing, home range, and migration studies. Public health dangers, although largely imaginary, seriously limit the use of this technique and are discussed.