The gene of nitrous oxide reductase (nosZ) and entire region of nos gene cluster from Pseudomonas sp. strain MT-1, which is isolated from the sediment of Mariana Trench, was identified. Gene organization in nos gene cluster is similar to that of other bacteria, and nosZ was highly homologous to nosZ of other Pseudomonads. The remarkable property was the existence of a gap from Arg-302 to Ser-309 (numbered by NosZ of P. aeruginosa), which might be a feature of marine bacterial NosZ. It is interesting that nosZ of the strain MT-1 is not related so closely to P. stutzeri, although 16S rDNA of the strain MT-1 is highly homologous to that of P. stutzeri. The existence of nosZ gene in the strain MT-1 indicated the presence of a complete denitrification (nitrate to N(2)) in the Mariana Trench. This is the first report on genetic analysis of denitrification in the deep sea.