Partial pressure of CO, (pC02) and its isotopic composition (513CairC02) were measured in Postojnska jama, Slovenia, at 10 locations inside the cave and outside the cave during a one-year period. At all interior locations the pC02 was higher and 513CairC02 lower than in the outside atmosphere. Strong seasonal fluctuations in both parameters were observed at locations deeper in the cave, which are isolated from the cave air circulation. By using a binary mixing model of two sources of C02, one of them being the atmospheric C02, we show that the excess of C02 in the cave air has a 513C value of-23.3 plus or minus 0.7 %o, in reasonable agreement with the previously measured soil-C02 513C values. The stable isotope data suggest that soil C02 is brought to the cave by drip water.