The agonistic and sexual behaviour of the Cyprinodont fish Aphyoseznion bivittatum is described. Both males and females develop colour patterns which are useful in identifying the motivational state of the fish and these are also described. Aggressive encounters between males are prolonged and highly ritualised. They result in dominant - subordinate relationships being set up and this is probably associated with a hierarchical structure of social organisation in the wild. The data was analysed to show differences in the sequences of aggressive behaviour of dominant and subordinate males. The possible adaptive values of the differences found are discussed. The sequence of courtship behaviour is relatively simple due to the limited behavioural repertoire of females.