Phorids parasitizing Coccinella septempunctata, the seven spot ladybird, attend host prepupae and parasitize them at the point of ecdysis to the pupal stage. In this system, oviposition rates would be maximized through the choice of older pre-pupal hosts in preference to young ones. Field study revealed that old pre-pupal hosts were indeed more likely to be attended by phorids than young pre-pupae. We tested the hypothesis that this was due to a preference by simultaneously offering phorids an old and a young prepupal host in a choice test. The results suggest that phorids do indeed distinguish between host prepupae on the basis of age, choosing the older prepupa.