Some territorial animals discriminate among neighbours and strangers based on individual differences in acoustic signals. Male North American bullfrogs, Rana catesbeiana, display this form of discrimination based on individual variation in advertisement calls. In this study, we investigated the acoustic basis of neighbour-stranger discrimination to determine how individual identity might be encoded by particular properties of bullfrog advertisement calls. We analysed patterns of within-male and between-male variability in 1078 bullfrog advertisement calls recorded from 27 territorial males. All call properties that we examined varied significantly among males. However, fundamental frequency and dominant frequency showed the lowest within-male variation and the highest repeatability between two recording sessions, and both properties were highly correlated with the first canonical root from discriminant function analyses, which typically accounted for 70-80% of the variability between males. We suggest that neighbour-stranger discrimination in bullfrogs is partially mediated by between-male differences in the spectral or fine temporal properties of advertisement calls.