An obligately piezophilic strain was isolated from an amphipod crustacean obtained in the Challenger Deep region of the Mariana Trench during the DEEPSEA CHALLENGE expedition. The strain, MTCD1T, grew at extremely high hydrostatic pressures, with a growth range of 80–140 MPa (optimum, 120 MPa) at 6 °C. Phylogenetic analyses based on the 16S rRNA gene sequence indicate that it is closely affiliated with the genus Colwellia . Comparative 16S rRNA gene sequence analyses revealed 95.7, 95.5 and 95.2 % similarity to Colwellia maris ABE-1T, Colwellia piezophila Y233GT and Colwellia psychrerythraea ATCC 27364T, respectively. The major cellular fatty acids were C16 : 1, C16 : 0 and C22 : 6 (docosahexaenoic acid), and the sole isoprenoid quinone produced was ubiqinone-8. DNA G+C content was 48.6 mol%. The strain was positive for oxidase and catalase activities. Based on the results from this study, strain MTCD1T is a novel Gram-negative species of the genus Colwellia , and the name Colwellia marinimaniae sp. nov. (type strain MTCD1T=ATCC TSD-5T=JCM 30270T) is proposed. It is the most piezophilic organism yet described.