Seed-micromorphology of 31 species of Ferocactus was investigated in order to determine whether it would suggest relationship where gross morphology did not. A remarkable correlation between characters of the hilum-micropylar rim (HMR) and fruit is reported and used to circumscribe two sections within subgenus Ferocactus. Bisnaga Orcutt is resurrected for the section with juicy fruits and shiny seeds with narrow HMR. F. robustus and F. flavovirens each have the least derived suite of characters within their respective sections. The most derived species have tabular-concave testa-cells and are grouped separately on this basis. The determination of species limits may also be aided by investigation of seed morphology.