The structures of polyhydroxyalkanoic acids (PHA) containing long chain saturated and unsaturated monomers have been determined by gas chromatography-mass spectrometric (GC-MS) analysis, Trimethylsilyl (TMSi) derivatives of 3-hydroxyalkanoic acid methyl esters of PHA have been identified by comparing their retention times to those of chemically or biologically synthesized standards. From the mass spectra of the TMSi derivatives, the characteristic fragment of m/e 175 which was due to cleavage alpha to the derivatized hydroxyl group revealed a 3-hydroxyl functional group, and the molecular ion-related fragments of m/e [M-15], m/e [M-31] and m/e [M-73] clearly reflected their chain length. Differences in position of the unsaturated functional group caused a shift in the base peak and changes in the fragmentation pattern. Structural determination of a bacterial PHA by TMSi derivatization followed by GC-MS analysis can be routinely applied to a simple and rapid identification of isolated strains which can accumulate a PHA containing long chain saturated and unsaturated monomers. The Pseudomonas strains which could accumulate this functional PHA were isolated on the basis of GC-MS analysis, and the contents and compositions of PHA synthesized by the strains isolated were also determined.