Individuals of many territorial species discriminate between familiar territorial neighbors and unfamiliar strangers based on individual differences in acoustic signals. Many anuran amphibians are territorial and use primarily acoustic communication during social interactions, but evidence for acoustically mediated individual discrimination is available only for one species. As a first step in research designed to investigate individual discrimination in a second species of territorial frog, we examined patterns of within-male and among-male variability in 198 advertisement calls of 20 male green frogs, Rana clamitans. The aim was to determine which acoustic properties could be used by males to recognize their territorial neighbors and to estimate limits of reliability afforded by these properties for identifying different neighbors. All of the call properties that we examined exhibited significant inter-individual variation. Discriminant function analyses assigned between 52% and 100% of calls to the correct individual, depending on sample size and the call properties included in the model. This suggests that there is sufficient among-male variability to statistically identify individuals by their advertisement calls. The call properties of fundamental frequency and dominant frequency contributed the most towards discrimination among individuals. Based on their natural history and behavior and the results reported here, we suggest that male green frogs likely discriminate between strangers and adjacently territorial neighbors based on individual variation in advertisement calls.