A sonographic analysis of the structure of fox vocalisations, based on 512 adult and 73 cub vocalisations obtained from archive recordings, was combined with field data on the vocal behaviour of an urban fox population. Calls were described quantitatively by six variables: duration, lowest and second lowest frequency bands (from sonagrams), highest and second highest peak frequencies (from power spectra) and the number of components. They were separated into 20 call types, eight of which were cub vocalisations. Call types were used singly or in combination, and some gradation between particular call types was apparent. Hypotheses regarding call function were generated based on the matching of acoustic properties with their seasonal occurrence and the socioecological pressures acting on foxes at different times of the year. Calls that were structurally suited to agonistic and contact functions were found to be significantly more common during the winter, the time of mating and dispersal, when foxes move over greater areas.