The detrimental effects of brother-sister and nephew-aunt matings on the colony foundation success of Bombus terrestris (Linnaeus, 1758) queens were investigated under controlled laboratory conditions. Among 86 hibernated queens, 43 were mated with randomly selected males from the same colony (inbred group), and 43 were mated with unrelated males from other colonies (outbred group). Nests were observed twice a week, and some developmental determinants of colony quality were recorded. The results suggest that inbreeding adversely affects the time taken to initiate a colony, the time taken for the first male to emerge, the number of workers in the first brood, and the total number of workers and young queens. The percentage of the queens that produced at least 50 workers significantly differed between the inbred and outbred groups (13.95 and 62.79 %, respectively). In the inbred group, 48.83 % of the queens produced both workers and males in their first brood, versus 4.65 % in the outbred group. Although the total number of individuals in the first brood was not significantly different for the two groups, the inbred group produced a higher proportion of males in the first brood (43.99 versus 12.74 % in the outbred group). Thus, inbreeding has a substantial deleterious effect on captive B. terrestris populations, so it is necessary to maintain genetic diversity and prevent inbreeding when aiming for the sustainable laboratory rearing of B. terrestris.