1. 1. Land crabs with a respiratory mask were run on a miniature treadmill at controlled speeds. O 2 consumption ( dot V O 2 ) and CO 2 production measurements showed that the aerobic response to exercise was slow. 2. 2. Steady-state O 2 consumption was not attained during 10 min runs nor when they ran freely for 20 min in a treadmill respirometer. Recovery from exercise was prolonged in both cases. 3. 3. This indicates that this species, like another land crab, Cardisoma guanhumi, relies heavily upon anaerobic fermentation, phosphagen and O 2 stores to support its energetic demands during running. 4. 4. Aerobic compensatory devices were evident also. Peak O 2 consumption values did tend to rise with increased velocity of running as did the area under the dot V O 2 curve during running and recovery. 5. 5. Both ventilation volume and Oz extraction also rose during running; the latter seemed to be the most important of the two respiratory compensatory mechanisms. 6. 6. Circulatory adjustments were evident. Heart rate showed a rapid rise with exercise and the O 2 pulse (the amount of O 2 delivered per heart beat) increased directly with O 2 consumption as well. 7. 7. Energetic considerations suggest that running with a mask may be slightly more demanding than free running. 8. 8. Estimation of an energetic cost of transport for this 53 g crab suggests it is comparable to that of quadrupedal and bipedal vertebrates of a similar size.