When 117 strains of Yersinia were grown in serial broth culture at 22 or 30C, 44 (38%) formed a broad-spectrum, mannose-resistant hemagglutinin (MR/Y-HA) associated with thick (8nm) channelled (MR/Y) fimbriae; eight other strains (7%) formed a narrow-spectrum, mannose-resistant hemagglutinin (MR/K-like HA) associated with thin (4nm) non-channelled (type 3-like) fimbriae. The distribution of these two fimbrial hemagglutinins in different Yersinia species is discussed and their properties are compared with those of other reported mannose-resistant hemagglutinins. The thick fimbriae of four Yersinia species were antigenically similar as judged by immunoelectronmicroscopy. Likewise, the thin fimbriae of three Yersinia species were antigenically similar, though different from the thick fimbriae.