The anaerobic biodegradability of four bioplastics powders (125–250 μm), polycaprolactone (PCL), poly(lactic acid) (PLA), polyhydroxybutyrate (PHB) and poly(butylene succinate) (PBS), were compared at 37 °C under aquatic conditions using our previously reported evaluation system (the modified ISO 13975 method). The bioplastics were ranked PHB >> PLA > PCL in terms of anaerobic biodegradation rate. The biodegradability of PHB, PLA, and PCL was 90% in 9 days, 29% and 49% in 277 days, and 3% and 22% in 277 days, respectively. PBS could not be anaerobically biodegraded by the sludge used in this study. The eubacteria and archaea participating in anaerobic bioplastic biodegradation were detected using the RT-PCR DGGE method. Some eubacteria participating in anaerobic biodegradation of PCL and PLA were of unique character. Eubacteria involved in anaerobic biodegradation of PHB were 98% homologous with Arcobacter thereius, Clostridium sp, and Clostridium sp, respectively. The eubacteria involved in the anaerobic biodegradation of PLA were 100% identical to Xanthomonadaceae bacterium and Mesorhizobium sp.; however, the other three bacterium were poorly related (<95% similarity) to any of the cultured-bacterium or database sequences. Anaerobic biodegradation of PCL was conducted by eubacterium with 98% similarity to A thereius, and by five organisms with poor similarity (≤95%) to any of the cultured-bacterium or database sequences. Archaea involved in anaerobic biodegradation of PLA and PCL were 97% and 95% identical to Methanosaeta concilii and Methanobacterium petrolearium. Only faint bands were observed in the RT-PCR DGGE profile of the archaea generated during PHB anaerobic biodegradation.