Behavioural interactions are often analysed in terms of their costs and benefits to the actors [Hamilton, (1964) J. Theor. Biol.7 1–16; Gadagkar, (1993)Trends Ecol. Evol.8 232–234; Foster et al., (2001)Ann. Zool. Fenn.38 229–238]. Using the bumblebee Bombus terrestris, we wish to distinguish between two possible determinants of interaction behaviour between conspecifics, namely kin-directed behaviour that reflects genetic distance between individuals, or, alternatively, interactions guided by a functional distance between individuals, specifically, with respect to disease susceptibility. We find no relationship between contact rate of individuals and the genetic distance of their respective colonies. Interestingly, we do find a significant negative correlation between contact rate and the distance between the two colonies in susceptibility to a spectrum of parasite strains. This cannot be explained by either of the a priori alternatives so we propose two further testable hypotheses to explain our results.