The presence of impaired thermoregulatory and fluid balance responses to exercise in older individuals is well established. To improve our understanding on thermoregulation and fluid balance during exercise in older individuals, we compared thermoregulatory and fluid balance responses between sexagenarians and octogenarians during prolonged exercise. Forty sexagenarians (60 ± 1 year) and 36 octogenarians (81 ± 2 year) volunteered to participate in a 30-km march at a self-selected pace. Intestinal temperature (T in) and heart rate were recorded every 5 km. Subjects reported fluid intake, while urine output was measured and sweat rate was calculated. Octogenarians demonstrated a lower baseline T in and a larger exercise-induced increase in T in compared to sexagenarians (1.2 ± 0.5 °C versus 0.7 ± 0.4 °C, p < 0.01), while maximum T in tended to be higher in octogenarians (38.4 ± 0.4 °C versus 38.2 ± 0.3 °C, p = 0.09). Exercise intensity (70 ± 11 % versus 70 ± 9 %) and exercise duration (7 h 45 min ± 0 h 57 min versus 7 h 24 min ± 0 h 58 min) were not different between octogenarians and sexagenarians. Octogenarians demonstrated lower fluid intake (251 ± 97 mL/h versus 325 ± 125 mL/h, p = 0.01) and urine output (28 ± 22 mL/h versus 52 ± 40 mL/h, p < 0.01) compared to sexagenarians. Furthermore, the sweat rate tended to be lower (294 ± 150 mL/h versus 364 ± 148 mL/h, p = 0.07) in the octogenarian group. Sodium levels and plasma volume changes were not different between sexagenarians and octogenarians (all p > 0.05). These results suggest that thermoregulatory responses deteriorate with advancing age, while fluid balance is regulated appropriately during a 30-km walking march under moderate ambient conditions.