Context . Estimates of the sex ratio of a population are a common summary statistic used for ecological studies and conservation planning. However, methods to determine the sex ratio often ignore capture probability, which can lead to a perceived bias in the sex ratio when the sexes are detected at different rates.

Aims . To illustrate the bias from conventional count-based analysis methods for determining sex ratio by comparison with analytical methods that include capture probability.

Methods . Closed-population mark–recapture analysis was used to determine the population size of each sex within a population of green and golden bell frogs (Litoria aurea). This was then compared with the traditional count-based methods of estimating sex ratio to determine the effect of incorporating capture probability on the sex ratio estimate.

Key results . More males than females were detected during surveys, producing a male-biased sex ratio when there was no incorporation of capture probability. Mark–recapture results indicated a similar population size between the two sexes, suggesting that the sex ratio is closer to even.

Conclusions . Methods to estimate sex ratio that incorporate capture probability can significantly reduce the bias obtained from count data.

Implications . We suggest that population studies must incorporate capture probability to determine the sex ratio of a population.