In the Ruff two groups of males can be distinguished: independent males and satellite males. This classification is based upon differences in territoriality and behaviour, and is highly correlated with differences in the nuptial plumage. Independent males are mostly dark coloured; their behaviour contains much fighting and related activities. They can be subdivided into resident males and marginal males. Resident males defend territories (residences), marginal males do not. Satellite males are mostly white. They behave peacefully and do not defend a territory. Satellite males have access to the residences on an arena (cfr. HOGAN-WARBURG, 1966). In this study an attempt has been made to throw some more light upon the behavioural dimorphism (independent males versus satellites). On the one hand I tried to analyse the causation of this phenomenon, on the other hand I was interested in its biological significance. I have tackled these problems by means of an examination of film material, and by a quantitative study of the behaviour on different arenas (leks) in the field. The film supplied me with detailed data about the behaviour of resident males, satellite males, and females in different external situations. On the basis of these data it was possible to analyse the temporal associations between the various behavioural units, for this purpose divided into "postures", "locomotion sequences", "action sequences", and "actions". The observed relations were largely similar for the three categories of individuals. For this reason I was able to compose one model for the hypothetical causal mechanism underlying the behaviour of resident males, satellite males, and females. The display of postures was supposed to be controlled by the values of two variables, the display of the other units by the value of the same variables and directly by the external situation. The values of both variables depended on external stimuli and on the internal situation (partly determined by previous external stimulation) I could not obtain evidence for the existence of different mechanisms underlying the different groups of behavioural units serving one function, such as aggression, flight, and sex. These groups of behavioural units were fully integrated in the entire causal system. They each appeared at distinctive combinations of values of both variables. Aggressive and sexual behaviour appeared each at only one cluster of combinations, however, protective behaviour appeared at three clusters. The suitability of the drive concept for both variables has been discussed. Normal behaviour of the three categories distinguished seemed to correspond to category-specific ranges of both variables. Female behaviour represented the one extreme, resident male behaviour the other one, and satellite behaviour was mainly intermediate. This intermediate behaviour has been ultimately ascribed to the light plumages of satellites and the genetical factors connected with that; the lack of a territory and distinct components in the behaviour of resident males towards satellites have also been emphasized as causal factors. I presented arguments for the hypothesis that the status (independent or satellite) of an individual depends on genetical and environmental factors. The importance of genetical factors appears from the correlation between behaviour and plumage colour. The influence of the environment follows from (a) the inconstancy of the status of some individuals (particularly young ones), and (b) the relation between status of an individual and plumages of the other individuals present on the same arena. It has been emphasized that some of the plumage colours occur exclusively in satellite males (satellite plumages), and others exclusively in independent males (independent plumages). Males with untypical plumages (intermediate colours) do not necessarily belong to one and the same category. They generally belong to the independent category when there are many males with satellite plumages on the arena, and relatively often to the satellite category when independent plumages are common. This phenomenon could not be ascribed to differential reproduction in previous seasons; it seemed to be connected with migration and changes of status, probably caused by low copulation frequences. It is argued that males with independent plumages are homozygotes with respect to a particular gene, that males with satellite plumages represent the same condition but with alternative alleles, and that males with untypical plumages are heterozygotes. This implies that both the behaviour and the plumage are influenced by one pair of genes (pleiotropy). Resident males can enlarge their reproductive output when tolerating satellite males on their residences in particular situations. The presence of satellite males on residences promotes the attraction of females. However, the presence of satellites has no influence on the duration of female visits, moreover, it has a negative effect on the copulation frequency of resident males. It could be demonstrated that the behaviour of resident males towards satellites is either tolerant or intolerant. Intolerant behaviour occurs when the resident male acquires a high density of female visits on his residence; it also occurs during and after high copulation frequencies of the male concerned. In all other circumstances tolerant behaviour will be shown. Both kinds of behaviour regularly alternate with a period of some days up to some weeks. This causes a large fluctuation in the density of satellite visits to the residence. The density of female visits also fluctuates, but less extremely. The other factors influencing female and satellite visits to residences have been analysed. This enabled me to formulate a model by which the oscillations in the densities of female and satellite visits could be simulated. Furthermore the effects of satellites on copulating of resident males could be studied. Satellite males...