The degradation of cellulose acetate (CA) filters was studied in natural environments. The degradation of CA filters buried in soil at 7 different places was slow, and the average loss in their weight was approximately 10% after 14 months. The decrease in weight and tensile strength of CA filters was accelerated in alkaline soil. In both seawater and river water, on the other hand, the degradation rate of the CA filters was considerably faster than in soil, and they disappeared within a year. The CA filters immersed in seawater disappeared faster than in river water. The filters without plasticizer were observed to degrade a little faster than with plasticizer in both soil and water. The dependence of the degradability of CA on the degree of substitution (DS) was investigated by the use of CA films. The CA films of lower DS were superior in degradability to those of higher DS. The factors governing the degradation of CA filters in environments and the relationships between these factors and the properties of the CA filters will be discussed in some detail.