It is well documented that resting body temperature (Tb) of mammals, measured under standard laboratory conditions, differs for the three major clades, with average Tb lowest for monotremes (30.9°C), intermediate for marsupials (35.5°C), and highest for placentals (36.6°C). It is not clear whether these phylogenetic differences remain for mammals when active in the field, but the increasing use of small implantable temperature loggers enables this question to be addressed for resting, active and maximum Tbs. Monotremes in the field have a lower maximum active Tb (about 35.3°C) than both marsupials and placentals, which do not differ (about 39.3 and 39.9°C respectively). Similarly, active Tb is lower for monotremes in the field (about 33.1°C) than both marsupials and placentals, which do not differ (about 37.5 and 37.8°C respectively. Resting field Tb is different for monotremes (about 31.2°C), marsupials (about 33.9°C), and placentals (about 35.5°C). The similarity of maximum active and active Tbs for marsupials and placentals in the field suggests that Tb-dependent physiological differences between the two clades when resting in the laboratory (such as metabolic rate and evaporative water loss) will be reduced but probably not eliminated during activity in the field.