The major groups of marine and estuarine macroinvertebrates of the tidal marshes of the northern Gulf of Mexico are described in this guide for students, taxonomists and generalists. Information on the recognition characteristics, distribution, habitat, and biology of salt marsh species from the coelenterate, annelid, mollusk and arthropod phyla is given. The taxonomy of some species is clarified. Most of the species discussed are illustrated. Binomial scientific names are used in the guide, but common names are provided when known. The authorities who originally named and described the species and the year in which it was described are credited. A bibliography of related publications is given. Taxonomic keys and schematic drawings of some polychaete worms, mollusks, and crustaceans associated with tidal marshes are also provided. A glossary contains frequently used terms associated with organisms of the tidal marsh. (ML)