The presence and phylogeny of methylotrophs, including methanotrophs, in a deep-sea sediment of a tropical west Pacific Warm Pool site WP was investigated by molecular marker-based analysis of mxaF, pmoA and archaeal 16S rRNA genes. MxaF amino acid sequence analysis revealed that microbes belonging to the alpha-Proteobacteria and most related to Hyphomicrobium and Methylobacterium were the dominant aerobic methylotrophs in this deep-sea sediment; also, a small percentage of type II methanotrophs, closely related to Methylocystis and Methylosinus, were detected in this environment. On the other hand, the use of a pmoA gene marker could not demonstrate the presence of any methanotrophs in this environment, suggesting that the mxaF gene probe is a more suitable marker in this deep-sea sediment for the detection of methylotrophs (including methanotrophs). mxaF quantitative polymerase chain reaction results showed that the west Pacific WP sediment contained approximately 3x10(4-5) methylotrophs per gram sediment, 10-100 times more than the samples collected from several other deep-sea Pacific sediments, but, on the other hand, about 10 times less than the amounts present in samples collected from rice and flower garden soil. Archaeal diversity as analyzed by 16S rRNA gene sequences indicated that a non-thermophilic marine group I crenarchaeote was the major archaeal group present in the west Pacific WP.