Viability of C. trachomatis in thermal waters collected from 5 spas was examined by inclusion forming ability in HeLa cells. In the acid springs (pH 2.0-2.2), the organisms lost their infectivity within 0.5hr when they were incubated at 37°C and no inclusion were detected when incubated at 42°C. In weak alkaline springs (pH 8.0-8.8), the organisms could be detected after incubation at 37°C for 2hr or at 42°C for 1hr, but number of inclusions were significantly decreased.