The fourth Zoological Catalogue of Australia volume on Malacostracan Crustacea catalogues over 1200 species of amphipods (sea fleas, scuds and the beach and land hoppers), cumaceans and mysidaceans (opossum shrimps). This catalogue brings up-to-date taxonomy of all known species in the Australian fauna and is particularly important for the cumacean and mysidacean faunas which have not been previously catalogued. For each species there is information on primary synonymies, status and location of type material, type localities, general distribution and ecology. In addition, new diagnoses for all families and subfamilies are provided as well as references to key literature. These small cryptic crustaceans are common in all marine and some terrestrial habitats. Amphipods are important herbivores on shallow water algae and on the sea bottom they are predators, detritivores and significant scavengers. They contribute to fouling communities on wharves and boat bottoms and are a major part of all sea bottom communities. Cumaceans are particularly diverse in Australian sea bottom communities and mysidaceans occur in shallow waters around our coasts.