Conventional census techniques are inappropriate for estimating numbers of nocturnal animals such as the spotted hyaena (Crocuta crocuta). A technique based on attracting animals by taped sounds to calling stations is described. A simple independent experiment to estimate the probability of response to the sounds, as well as a probability model for estimating the population size in a given habitat based on the response counts, is proposed. The model allows for the possibility that some animals may not respond to sounds. The estimate of the population size has a simple form and the sensitivity of the estimate to changes in the response probability can be assessed. The model allows statistical comparisons to be made between different habitats or different times. Likelihood-based confidence intervals can be found for all estimates. The results of surveys in the Kruger National Park illustrate the technique which can be applied in different areas and possibly to other species.