The development of agonistic behaviour and vocalization in the croaking gourami was studied from hatching to four months of age, by which time fish became sexually mature. Initial interactions started when fry were 11 days old and consisted of Approach and Flight in a feeding context. More complex threat patterns appeared during dyadic encounters as fish grew older. Lateral Display (spreading of median fins in a lateral position) first occurred during the third week, Circling shortly afterwards and Pectoral fin beating when fish were seven weeks old. Rapid pectoral fin beating was first accompanied by sound emission at eight weeks. Initially, croaking sounds were composed of a series of single pulses, each one produced by one pectoral fin. Later single pulses gave way to double-pulsed bursts. The mean number of single and double pulses increased from 4.4 to 6.0 per croak and mean pulse period from 27 to 35 ms 1om initial sound production until maturity. Furthermore sound pressure increased significantly with growth of the fish. The dominant frequency of croaks decreased from 3600 to 1500 Hz with increasing size (15- 37 mm standard length). After vocalization was established Frontal Display, Mouth Biting and Appeasement Behaviour occurred at the age of ten weeks. The development of agonistic behaviour is mainly age and not size dependent in croaking gouramis, although growth-rates differed widely. Unlike behavioural patterns sound characteristics were clearly correlated with body size. To our knowledge this is the first study investigating ontogeny of vocalization in fish. Results in T. vittata demonstrate that sound production develops simultaneously with agonistic behaviour patterns and thus independently of sexual maturation.