Extreme climatic conditions constitute a challenge for the adaptability of fattening pigs. Heat stress poses major problems for pig production and welfare because it leads to a prolonged fattening period, reduced fertilization success and higher overall mortality. Indicators need to be established for farmers to be able to recognize heat stress at an early stage and to respond by providing cooling measures. In this study, several behavioural and physiological parameters were measured in 72 fattening pigs of three weight classes (40–100kg) during one summer month. The measured ambient pen temperature ranged from 17 to 30°C. The core body temperature (n=7) measured with a vaginal logger showed large inter-individual variability, and no rise was detectable with increasing ambient temperature, probably because the highest ambient temperatures were still moderate and lasted for only a short period. The respiration rate (n=24) was highly variable and increased only in the heaviest pigs at 26–27°C, whereas the body surface temperature (n=24) increased with higher ambient temperatures in pigs of all weight classes. The measured behaviour (n=72) was largely independent of the ambient temperature. However, the percentage of pigs lying in sternal recumbency increased with rising ambient temperatures in the afternoon. To summarize, the results of this study suggest that respiration rate and skin surface temperature are reliable indicators of heat stress in fattening pigs.