The long-unknown crystal structure of Bi-III has been solved. It comprises
a body-centend-tetragonal (bct) "host" and a bet "guest" component made up
of chains that lie in channels in the host; the guest is incommensurate wit
h the host along the tetragonal c axis. Diffraction data for Sb-II reveal t
hat it too can be fitted with the same composite structure. The structures
of these two high-pressure phases of Bi and Sb are similar to those reporte
d recently in the alkaline-earth metals Ba and Sr.