Male house mice produce ultrasonic vocalizations (USVs) during courtship; however, it is unclear why males produce USVs and what information their calls communicate. In laboratory mice, male USVs are attractive to females 1,2. They appear to facilitate mating and coordinate copulation behavior 3,4 perhaps because USVs provide information about males' quality or compatibility. In our studies on wild house mice (Mus musculus musculus), we found that females can discriminate the USVs of unrelated males versus siblings 5. In this study we conducted spectrographic and temporal analyses on recordings of courtship USVs of wild males. We found evidence that males' vocalizations contain signatures of individuality and kinship. The individuality of males' USVs could be due to differences in the filter function of the vocal tract or differences of the vocal apparatus, which might directly influence the temporal and spectral features of vocalizations. Further studies are needed to determine the consistency of individual USVs over longer periods of time and across contexts, and whether the familial effects we found are due to genetic relatedness, social learning (imprinting), or both.