Significant changes in temperature and water chemistry were observed in Lake Yugama, a crater lake at Kusatsu-Shirane volcano, Japan, between 1988 and 1993. Heat, water and chloride budgets were evaluated with a box model. Until 1989, heat and chloride inputs into the lake were 3∼10 MW and 1∼3.5 tons/day, respectively; but these increased to 19–25 MW and 4–12 tons/day by 1990. The strong correlation between the Cl– concentration and lake water pH suggests input of a HCl solution or HCl vapor as the cause of the Cl– change. These changes coupled with the high level of seismicity observed around the volcano in 1990 seem to be consistent with the formation of fractures in the surrounding area of a cooling magma.