A Gram negative, rod-shaped mesophilic bacterium active for poly(L-lactide) (PLA) degradation was isolated through the enrichment culture and clear-zone method. The isolated strain was identified to be Bordetella petrii PLA-3 on the basis of 16S rDNA gene sequence analysis. B. petrii PLA-3 was active not only for the degradation of low-molecular-weight PLA but also for the degradation of high-molecular-weight PLA. The strain seemed to attack the crystalline part of PLA as well as the amorphous region. The PLA film incubated in compost inoculated with the isolated strain lost its weight more notably and exhibited a lower molecular weight than that incubated in the sterilized compost without living microorganisms. Moreover, the profile of the cumulative amount of CO2 after 20 days of burial in the sterilized compost and subsequent inoculation of the isolated strain into compost was nearly the same as that of CO2 evolved from PLA buried in compost with the isolated strain at the very beginning when the time was shifted by 20 days. This indicated that not only the abiotic hydrolysis but also the microbial enzymes of the strain contributed to the initial chain cleavage of PLA molecules and resolved the doubt that PLA molecules should be initially cleaved into very low-molecular-weight substances by abiotic hydrolysis to be subsequently absorbed into and biodegraded by microorganisms. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 67-74, 2010