Bat detectors are commonly used to monitor bat behaviour. Earlier research has suggested that there may be systematic differences in the response of different detectors to bat calls. Such differences would have important implications for the comparability of quantitative surveys conducted with bat detectors. The present study examines variability within and between brands of bat detector in accuracy of tuning, directionality and sensitivity to different types of bat echolocation call in bat detectors from three manufacturers. The consistency of results from a field survey incorporating the three brands in a standardised methodology are also examined. Significant differences were found within and between brands in directionality and sensitivity which would lead to bias in bat surveys. The implications of these findings for bat surveys are discussed, as are the design features of importance for species identification.