Shelly intracrystalline proteinaceous macromolecules isolated from forty four Recent terebratulide brachiopod species, covering all living superfamilies and two thirds of living families, have been compared using immunological techniques. Immunological data indicate that the examined species belong to one of the following four groups, which are also morphologically distinct: 1) Cancellothyridoidea, 2) "Terebratelloidea', 3) Terebratuloidea, and 4) a newly identified category (Kraussinidae, Megathyrididae, Macandreviidae, Ecnomiosidae). Immunological data clearly indicate that groups 3) and 4) form a coherent cluster, and that this cluster has a trichotomous relationship with the remaining two groups. -from Authors