There are similar to 20 caves of limestone origin in Zonguldak (Turkey). In this study, the results of atmospheric radon measurements performed for two caves are presented. These caves, Gokgol and Cehennemagzi, are open to tourism. Gokgol Cave is the longer, at nearly 3200 m in length. Cehenennemagzi is a pit-type cave with a total length of 85 m. The radon measurements were performed for 2 months between July 2004 and September 2004 using passive polycarbonate detectors. The mean radon concentrations were recorded as 1918.8 Bq m(-3) in Gokgol Cave and 657 Bq m(-3) in Cehennemagzi Cave. The maximum value corresponds to a site located 400 m from the entrance of Gokgol Cave. Mean effective dose values for tourists of 0.86 mu Sv per visit to Cehennemagzi Cave and 3.76 mu Sv to Gokgol Cave were obtained. These results show that protection against radiological hazards would not be necessary for visitors to these two caves.