A musty odor developed in the water of the northwest part of Lake Kasumigaura in May 1982. Chemical and biological environmental factors causing this musty odor were investigated. A musty odor compound in the lake water was isolated by the activated carbon adsorption and extraction procedure, and identified as 2-methylisoborneol by GC-MS analysis. The concentration of 2-methylisoborneol was 0.20μg·L^-1 in the lake water. A significant increase in the cell number of Synedra acus, a non-odoriferous alga, was observed during the period of musty odor occurrence, but the musty odor-producing algae were not present at that time. A large number of actinomycetes and a relatively high pH value were observed in the surface mud. It was suggested that the good growth of actinomycetes in the mud at the high pH value was responsible for the musty odor production.