A method for estimating the size of a population of mobile animals. It involves collecting a random sample of individuals, marking them, and releasing them. After allowing sufficient time for the marked individuals to disperse among the wider population, a second sample is taken. The proportion of marked individuals in this sample is assumed to be the same as the proportion of the original sample to the entire population; hence the size of the population can be estimated. The method assumes that marked and unmarked individuals have the same probability of being captured and that the marked individuals are representative of the population as a whole, e.g. in terms of viability, behaviour, and so forth.