Gregarines are apicomplexans and exclusively parasites of invertebrates. After the basic publication in the Traité de Zoologie by Grassé in 1953, this second edition is proposed to update the knowledge with information provided by new technologies such as electron microscopy, biochemistry and molecular biology and to enlighten their high diversity of adaptation to invertebrate hosts living in a diversity of biotopes. Morphological features, life cycles, host-parasite interactions and taxonomical data are most informative for the understanding of the evolution of the phylum Apicomplexa.