The behaviour of the mark-recapture estimators of Petersen, Bailey (triple catch) and Jolly and Seber are examined theoretically and empirically by means of simulation techniques. The correlation between the parameter [Formula: see text] and its associated variance is shown to be significant for all the estimators. This correlation makes the estimated variance an insensitive measure of the accuracy of the estimate except at very high sampling intensities. Such sampling intensities are rarely achieved in experimental work and so the method of mark-recapture must be considered of very limited use. At the sampling intensities necessary to give a coefficient of variation of less than 0.05 it does not seem likely that the correlation between [Formula: see text] and its variance will produce serious underestimation but the minimum confidence limits [Formula: see text] are recommended.