Book excerpt: This comprehensive guide encompasses a total of 473 species of mesopelagic fishes (i.e. those fishes residing primarily between 200 and 1 000 m depth during daytime) that are known to occur in the western Indian Ocean. It aids in their accurate identification, offering dichotomous keys to all taxa. Species are treated in detail, with accounts including the scientific name, FAO common name in English (where available), other useful characters, size, a distribution map, and one or more illustrations. Captions and arrows enhance user-friendliness for to both specialists and non-specialists in the field of ichthyology. In the broader context of marine ecology, mesopelagic fishes hold a pivotal role as integral components of the marine food web, serving both as predators and prey for various species. This guide not only enriches our understanding of these fishes but also contributes to our knowledge of the western Indian Ocean's diverse ecosystem. Beyond species identification, it sheds light on the interconnectedness of marine life and underscores the importance of conservation efforts in safeguarding this delicate balance. This guide serves as an invaluable resource for in-depth exploration and research into these remarkable fishes, offering a comprehensive tool for studying their diverse characteristics, behaviours, and ecological significance, essential elements for sustainable management and conservation practices.