To investigate whether great tits (Parus major) avoid roosting in flea-(Ceratophyllus sp.) infested nest boxes, roost-site selection in relation to naturally occurring flea numbers was studied in two nest box areas on Gotland, Sweden. In one area (A), every second nest box was provided with an old tit nest, while the rest were cleaned and left empty. In the other area (B), a choice was offered between empty boxes and boxes containing old collared-flycatcher (Ficedula albicollis) nests. Great tits clearly preferred to roost in empty boxes in area A, but not in area B. However, there was no statistical evidence for differential preference in the two areas — a result consistent with the finding that there was no difference in flea numbers between old tit (area A) and flycatcher nests (area B). The interpretation that roost-site choice was affected by parasite load was further supported by the fact that individuals roosting in boxes with an old nest preferentially chose boxes with low flea numbers.