In the present work, we have analyzed aggressive patterns shown by rats reared in different social situations. The mixed colonial group shows significantly lower scores in those behaviour patterns which are grouped in the Physical Attack factor, and mixed colonial rats are the ones that score highest in those behaviour patterns which are grouped in the Expectancy factor. Thus, these animals are the ones that show more Expectancy and less Physical Attack. The presence of female rats in mixed colonies would increase the Expectancy. In a situation where there is no artificial manipulation (male colonies) or in which the intervention is small (mixed colonies) correlation between those factors (Physical Attack and Expectancy) is high, that is to say, these factors are close to one another. But when there is a clearer interference, like social isolation, the two factors separate and reveal themselves to be independent from one another. Thereafter, we have analyzed the possible influence of individual differences in a low-frightening Open Field in aggressive behaviour. On the Physical Attack factor the interaction Ambulation-Defecation is nearly significant and the correlations of Ambulation and Defecation with Physical Attack are positive and negative, respectively and nearly significant in mixed colonies but not in isolated rats. We suggest in accordance with Karli, Vergnes, Eclancher, Schmitt and Chaurand (1972) that in an appetite situation the less fearful rats (low defecatory and high ambulatory rats) attack with higher frequency and intensity. © 1988.