The common brushtail possum (Trichosurus vulpecula) is usually a strictly monotocous species, presenting one offspring at a time. Two concurrent offspring is a rare occurrence, but twin pouch young or young spaced one oestrous cycle apart have been reported. We investigated the biology of urban T. vulpecula populations within the Sydney metropolitan area between November 2002 and April 2005. As part of these studies, we trapped one adult female T. vulpecula with two young. Analysis of morphological data from the young and development of the female's mammary gland suggest that only one young was her offspring. The results of genetic analysis support this conclusion. Herewith, we describe an apparent case of adoption in T. vulpecula.