This paper deals with the description of the behaviour of Limnotilapia dardennei (Boulenger, 1899), a mouthbrooding cichlid of Lake Tanganyika. Each of the elementary actions belonging to feeding and comfort behaviour, and aggressive, submissive and reproductive behaviour, is described in detail. Since the various patterns of markings this cichlid assumes may play an important role in its communication system, they are described and their possible significance is discussed. It is postulated that they express a territorial status, neutral mood, inferiority, surrender, fleeing mood and hiding mood. The mutual relationships of the behavioural elements described here and quantitative evidence for the postulated significance of the patterns of markings will be treated in subsequent papers. We give here a very detailed description of the behaviour of the species, because it is a necessity for any further research; for quantitative analysis as well as for an experimental approach.