Anderson’s mouse opossum is known only from one individual collected in 1954 (3), and several more specimens caught in the late 1990s (2), and thus very little is known about the biology of this incredibly rare animal. However, much can be deduced from studies of closely related species. It is likely to be nocturnal, and spend most of its time in trees (1). Like all marsupials, gestation is probably short, with females’ giving birth to poorly developed young and most of the development taking place during lactation (4). It is likely that reproduction is similar to that of Marmosa robinsoni, which gives birth to 6 to 14 young after a gestation period of just 14 days. The tiny young, measuring only up to 12 millimetres, attach themselves to the mother’s mammae where they may remain for around 30 days (5). Unlike many marsupials, female mouse opossums do not possess a pouch to protect the young as they develop (4). The young are so un-developed their eyes do not open until 39 to 40 days. It is likely that the young are completely weaned after around 65 days, and they may have an incredibly short life span of only one year (5). Marmosa species build nests for shelter, or use abandoned bird nests, holes in trees, or banana stalks. These nest sites are unlikely to be permanent; rather, the opossum will use whatever site is available as the sun begins to rise (5). Like M. robinsoni, it is likely that Anderson’s mouse opossum is insectivorous, with fruit also playing an important role in the diet (5).