A preliminary study on the sources of the earthy-musty odorants in Lake Donghu were conducted during June 1995-May 1996. Earthy-musty odors in the lake were mainly caused by 2-methylisoborneol (MIB). MIB showed three concentration peaks and a maximum of 320ng/L, which was over 10 times of its odor threshold. The higher the trophic level in the lake water, the higher the concentration of the earthy-musty odorants were found. The main source of MIB in Lake Donghu came from the secondary metabolites of the algal Phormidium tenue and the Actinomycets.