Excessive growth of Phormidium tenue and musty odor due to it have been observedrecent years in lake Kusaki located in Gunma, Japan. Variations of the algal growth and water qualities in the lake were observed for two years, and investigated a possibility of forecasting the excessive growth from a simple ecological model with known parameters which were generally reported in former studies. The field surveys showed that distributions of temperature and nutrient concentrationswere significant in the vertical direction but not in horizontal, therefore, a vertical one-dimmensional mathematical model consisted of the heat and mass balance equations was used. The model fairly evaluated the seasonal changes in water qualities of TN, TP, TOC, and Chl. a as well as the thermal stratification and turnovers. Although some modifications of the model should be necessary to evaluatethe P.tenue population exactly. the growth pattern which had two peaks in earlyand late summer in 1990 were estimated reasonably well by assuming a maximum specific growth rate of 2.5 d^-1.