We analyzed 18 years of data collected from a breeding colony of captive koalas Phascolarctos cinereus in order to identify factors regulating mating success and to generate expectations about the behavioral ecology of wild koalas. Short-term mate fidelity was associated with reproductive success, but familiarity appeared to diminish reproductive output with time. Male body mass had no effect on reproductive success. Reproductive output was highest when males were slightly older than females. Survivorship of joeys was not dependent on either male or female age. We suggest that sexual selection regulates koala mating tactics primarily in the context of non-agonistic male competition. We hypothesize that field studies will reveal that females in the wild probably capitalize on the link between male vocal and olfactory advertisements and male age as a mechanism to foster positive assortative mating by age.