Growth of pouch young from birth to first exit from pouch was recorded in female bare-tailed woolly opossums, Caluromys philander , a didelphid marsupial from French Guiana. An age-predicting curve was assessed for its use in determining the age of pouch young from length of body in free-living individuals. Increase in body mass of sucklings during pouch life indicates the level of maternal investment during lactation; reproductive effort appears to be low during the first 40 days of pouch life, then increasing to a higher rate in the last 40 days. Maternal investment during lactation is discussed in light of variations in energetic cost during lactation.