The gray short-tailed opossum (Monodelphis domestica) began tooth eruption at 32 days with the deciduous premolars (dp3 and dP3). All but 5 teeth completed eruption by 56 days, with only p3, m4, P3, M3, and M4 unerupted at that age. Eruption was complete by 134 days with eruption of P3 and M4. We found no significant differences between sexes in tooth eruption timing, but significant differences occurred among litters at some tooth positions. Timing and sequence of tooth eruption differed somewhat as seen in live young versus that seen in a series of skulls of known age. Live juveniles can be placed into a series of 6 age classes based on emergence of teeth through the gingiva, whereas juvenile skeletal material is best placed into age classes based solely on eruption of upper molars. Other systems of age classes used in didelphid marsupials that are based on sequence of eruption of P3 and M4 are not generally applicable because of variation in this sequence. In didelphids delayed eruption of I1 may be functionally related to weaning, whereas weaning and 1st reproduction are not obviously correlated with age class based on molar eruption.