Preferences for nest box size and position were tested in two groups of common marmoset, Callithrix jacchus jacchus. In a pre-test phase two, new, wooden nest boxes were introduced to each group alongside their old metal one, so that the animals could become familiar with them, and so that any pre-existing preference could be identified. In successive experiments: i) the old metal nest box was closed so that the marmosets had to choose a preferred box from the two new nest boxes; ii) the size of the preferred nest box was systematically reduced; and iii) in one of the groups the reduced nest box was restored to full size, but its position interchanged with the non-preferred box. In a further experiment, the position of the preferred, and then of the non-preferred box was raised to the highest point in the cage. The marmosets showed strong positional preferences which could not have been predicted without performing the tests. One of the groups also showed a strong preference for the high nest box. It was concluded that marmosets may exhibit preferences for their cage furniture, the basis of which may not be clear to us. These preferences should, however, be taken into account when designing cages to maximize the welfare of the animals.