We examined gametogenesis and auxospore development in the cymatosiroid diatom Brockmanniella brockmannii. Our mating experiments demonstrated that the clones were homothallic, self-compatible, produced two eggs per oogonium and four hologenous uniflagellate male gametes per spermatogonium. Auxospores grew free in the culture medium and the most external layer of the auxospore wall was made up of spinescent scaly elements in their early globular stage of development. The anisodiametric expansion of the globular auxospore was rather limited, at best proceeding only to a sub-globular stage. Our data suggest that the initial epivalves, nearly pennate-like in outline, developed within such auxospores curled up against the ventral side of the auxospore wall, not across its equatorial plane, and apparently without the help of rings, hoops or bands characteristic of pennate-type transverse perizonia. Thus, B. brockmannii showed a mode of initial epivalve development thus far unknown among diatoms. We propose that progressive silicification of the initial epivalve facilitated the breakdown of the sub-globular auxospore dorsal wall to accommodate for its straightening. The structural features of the spinescent scales on the auxospore wall suggest that Cymatosiraceae could be related to the mediophycean family Eupodiscaceae, although most current molecular phylogenies do not support a direct relationship.