Hydrogen and oxygen isotope ratios of meteoric waters in Okayama Prefecture collected in 1990 to 1992 are reported. The δD and δ18O values of precipitation collected at Okayama University, Okayama City, vary in the wide ranges from +20.0 to -123.0‰ and from 0.0 to -17.0‰, respec-tively. Precipitation in the rainy season (June and July) has distinctly lower δD and δ18O values than in the other seasons. The weighted averages for a year from July, 1991 to June, 1992 are δD=-53.0‰ and δ18O=-8.1‰ with a d parameter of 11.8‰.
River waters are uniform both in δD and δ18O values and they range from -47.8 to -58.7‰ and -7.9 to -9.1‰, respectively, with the average δD value of -51.9‰, δ18O value of -8.6‰ and d parameter of 16.9‰, which indicates the river waters of Okayama Prefecture are just on the meteoric water line of Japan. The difference in d parameter between the average precipitation and the average river water may be explained by significant contribution of precipitation with high d para-meters in northern Okayama Prefecture to the river waters. Most well waters show deviations from the meteoric water line of Japan, indicating kinetic isotope effects during evaporation. The Yubara hot spring waters have the same δD and δ18O values to the nearby Asahigawa river water.