To investigate patterns of thermo regulation in free-ranging and captive southern brown bandicoots Isoodon obesulus, we measured abdominal body temperature (T-b) of five free-ranging bandicoots over 42 days using implanted data loggers and T-b of three captive bandicoots over 3 months using implanted temperature-sensitive radio transmitters. Bandicoots in the wild had a mean T-b of 36.5 +/- 1.0 degrees C (range 33.4-39.8 degrees C) and showed a pronounced nychthemeral pattern with two distinct temperature phases. T-b increased at 13:30 +/- 2.6h each day and remained high for 10.65 +/- 2.07h, suggesting a crepuscular and early evening activity pattern. Daily T-b variation of I. obesulus would save considerable energy by reducing daytime thermo regulatory costs in the wild. Captive bandicoots had a similar mean body temperature (36.9 +/- 0.2 degrees C) and range (33.0 +/- 39.9 degrees C) as free-ranging bandicoots. However, the nychthemeral T-b pattern of captive bandicoots was different from free-ranging bandicoots, with a less pronounced daily cycle and the nocturnal rise in T-b occurring mainly at sunset and the daily decline occurring mainly at dawn. (c) 2006 Elsevier Ltd. All rights reserved.