We investigated the composting process of an aquatic plant for developing an effective method of their recycling. The changes in microflora were monitored by temperature gradient gel electrophoresis (TGGE) of the DNA fragments amplified by reverse transcription PCR from the 16S ribosomal RNA extracted from the compost. The decrease in the cellulose content of aquatic plant compost was found to be faster than that of other composts. During the phase when the specific activity of cellulase was high, the decrease in the cellulose content was rapid. In the TGGE analysis, a few bands were found that changed in intensity in relation to that of the specific activity of cellulase. The bacteria responsible for the bands were isolated and, based on the DNA sequences of the 16S rRNA, identified as Bacillus spp. For an effective composting process, we examined the use of the composted material as a seed. When the resultant compost was used as a seed for the next cycle of composting, the decrease in cellulose content during the second composting cycle occurred faster than that during the first cycle.