Intra-crystalline macromolecules isolated from the skeletons of nine species of Recent articulate brachiopods were compared by enzyme linked immunosorbent assay to assess the relationships within the Order Terebratulida. Immunological distance data indicated that the sub-division of this order into three suborders is not valid. Three major clusters were recognized within the Terebratulida which approximately correspond to recognized superfamilies, with the exception that the family Kraussinidae is placed within the Terebratulacea. The three lineages are more closely related to each other than was previously predicted, and yet within most lineages there is a greater degree of subsequent diversification than has hitherto been recognized. -from Authors