There are approximately 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, Gökgöl and Cehennemağzi, are open to tourism. Gökgöl Cave is the longer, at nearly 3,200 m in length. Cehenennemağzi 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 1,918.8 Bq m(-3) in Gökgöl Cave and 657 Bq m(-3) in Cehennemağzi Cave. The maximum value corresponds to a site located 400 m from the entrance of Gökgöl Cave. Mean effective dose values for tourists of 0.86 microSv per visit to Cehennemağzi Cave and 3.76 microSv to Gökgöl Cave were obtained. These results show that protection against radiological hazards would not be necessary for visitors to these two caves.