A colony of approximately 150 adult and infant guinea-pigs was studied in order to investigate the structure, causation and function of guinea-pig vocalizations. Behaviour and sound recording were made in a wide variety of contexts and sounds were analysed on the Kay Electric Sonagraph 6061 B. Sonagrams were measured, and on the basis of physical structure alone, 11 call types were distinguished. Behavioural records were examined and it was suggested that 5 functional categories existed. The significance of vocal communication in guinea-pig social behaviour was discussed.