Carnivore populations are difficult to measure by conventional methods. A new method was developed which involves catching one or more individuals from a population and injecting them with the isotope 65Zn. The radio-isotope appears in the feces; assuming that the proportion of labeled to unlabeled feces will equal the proportion of labeled to unlabeled individuals, the population size can be estimated. The method gives an accurate estimate for a population of captive badgers [Meles meles] of known size and was used in the field to estimate the sizes of wild badger populations.