Aims: To evaluate the ammonia-assimilating abilities of micro-organisms isolated from cattle manure composting processes and to determine the distribution of cultivable species of ammonia-assimilating micro-organisms in microbial communities during the composting processes. Methods and Results: Compost samples were collected from four stages of treatment. Trypto soya agar was used for the isolation of ammonia-assimilating aerobes. Many of the isolates showed high ammonia-assimilating ability in a medium containing basal components and a compost extract. Partial 16S ribosomal DNA sequencing showed that the cultivable species of highly efficient ammonia-assimilating isolates changed during the composting process. The community structure of micro-organisms and actinomycetes was analysed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). Two species of actinomycetes identified by PCR-DGGE coincided with those found among the cultured isolates. Conclusions: Ammonia-assimilating micro-organisms obtained by the cultivation method were not predominant in the microbial community during the composting process: however certain cultured actinomycetes were members of predominant species in the actinomycetes community. Significance and Impact of the Study: Ammonia assimilation by micro-organisms is one of the important mechanisms for ammonia retention in the composting process. Cultivable actinomycetes are a means for preventing ammonia emission from the composting process.