Individual variation in the susceptibility to handling stress (mechanical irritation) in laboratory-reared pupae of yellow mealworm, Tenebrio molitor, was studied by means of simple thermocouple twin set-up calorimeters. The stress condition was characterised by disturbances in normal rhythms of gas exchange and body stereotyped movements well distinguishable from calorimetric recordings. Recovering time from stress, induced by the device, was 10-15 min. In every newly established population, a portion of pupal individuals (13-15%) was highly sensitive to handling and these pupae lost significantly more water than the pupae exhibiting no symptoms of stress.