1. Mutual mate choice may be rare, occurring when both sexes invest heavily in reproduction, mating opportunities are abundant, and individuals differ in quality.
2. Mountain pine beetles, Dendroctonus ponderosae (Curculionidae: Scolytinae) appear to meet the conditions for mutual mate choice. We introduced males to females in breeding sites and observed the occurrence and speed of a male entering a female's gallery. We tested for consequences of mutual mate choice, namely condition-dependent choosiness and assortative mating.
3. Males were more likely to enter a female's gallery when the gallery was in a smaller tree with less resin production and when the gallery was larger. Female body size and condition did not influence the probability of entry. Larger males were less likely to enter a gallery than were smaller males, probably because of size-dependent choosiness rather than physical limitations.
4. Small males took longer to enter galleries of large females than of small females, whereas large males entered as quickly into galleries of large females as small females. This suggests size-dependent choosiness by females.
5. No assortative mating by body size was detected, probably because males appeared to choose on the basis of female-associated resources rather than on female traits.