In gregarious breeders, parents often use individually stereotyped vocalizations as a cue to relocate offspring. Harp seals aggregate in large colonies on pack ice during the whelping season. During the 11-day lactation period, females alternate between periods at sea and attending their pup. If they use vocal cues in the relocation process, individual variation in pup vocalizations would be expected. We recorded vocalizations, sex and age class for 91 individuals at whelping patches in the Greenland Sea. Pups produced three call types: tonal, pulsed and a combination of the two. Only tonal vocalizations were used for analyses. To explore individual variation in measured vocal parameters, we used classification trees: 43% of 4075 vocalizations were classified correctly according to individual. The first split was driven by the third peak frequency, splitting male pups from females. For females total duration produced most further splits, whereas the maximum frequency of the lower harmonic, the maximum frequency of the second harmonic and total duration caused splits between males. We correctly identified 55% of 42 female pups and 8% of 49 male pups based on vocal parameters. Calls were misclassified according to individual but never according to sex. Repeated measures of eight individuals over several age classes showed that 82% of 869 calls were correctly classified regardless of age. Alongside vision and smell, acoustic cues appear to be important in relocating offspring. Differences in vocal variability between sexes may reflect different selection pressures working on males and females. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.