The problem of unequal catchability among individuals in an open population is discussed in regard to estimates of population size. Properties of the Jolly–Seber (J–S) estimate are clarified and comparisons are made with minimum number alive (MNA) techniques which attempt complete enumeration of the population. The MNA is shown to be seriously biased under the assumption of equal catchability and to have a proportionately much larger negative bias than the J–S estimate under unequal catchability. Suggestions are made for future work.