The agonistic behaviour of juvenile Blennius pholis, L. was observed in tanks of 28 x 43 x 30 cm and 75 x 40 x 30 cm, the larger tanks thus having a floor area 2.5 times that of the smaller tanks. The smaller tanks contained two fish, the larger ones either two or five fish. Those containing five fish thus had a population density equivalent to the smaller tanks. Eight main elements of agonistic behaviour were observed. They were; advancing, threatening, charging, snapping, fleeing, chasing, retreating and submitting. It was found that charging, fleeing, and chasing were by far the most common elements performed. Charging, threatening and chasing were performed most frequently by dominant fish, fleeing and retreating by subordinate fish. Advancing was performed more or less equally by both dominant and subordinate fish. It is suggested that submission is a displacement activity. Size difference was the main factor deciding dominance and the intensity of aggression, but the onset of light, food, available space, and the activity of the fish concerned were also of importance. The size of the tank and the number of fish it contained had an effect upon the relationship between size difference and the intensity of aggression. In the smaller tanks the intensity of aggression was directly related to the difference in size between the two fish. This relationship was not as clear in the larger tanks. Territoriality in the normally accepted sense of the word was not observed, because the fish were not seen to defend any particular area of the tank against others. A hypothesis suggesting the existence of 'individual distances' is put forward, in which the fish are considered to defend a particular area of space around themselves. These individual distances fluctuate in size according to the state of the aggressive drive of the individual and the amount of space available to it for movement. An attempt is made to relate the behaviour observed in the laboratory to that occurring in nature.