ABSTRACTAgardhiella subulata(C. Agardh) Kraft et Wynne is a commonly collected erect, terete, alternately branched, fleshy red alga that is found both intertidally and subtidally in the western Atlantic Ocean, Caribbean Sea and Gulf of Mexico. A single periaxial cell is cut off from each succeeding axial cell in an orthostichous file. The supporting cells of carpogonial branches are homologous with auxiliary cells, being transformed from identical inner cortical cells. The auxiliary cell together with the three to four enlarged, darkly‐staining cortical cells pit‐connected to it form the auxiliary cell complex. Following fusion of a connecting filament to an auxiliary cell the diploid nucleus divides with one nucleus remaining near the point of fusion and the other migrating to the beaked portion of the auxiliary cell. Surrounding vegetative cells divide, forming files of cells that grow toward the auxiliary cell to form the involucre. Cells of the gonimoblast fuse with nearby cortical cells producing a placenta of vegetative and gonimoblast tissue. As the cystocarp matures the inner placental cells become vacuolate while at the periphery terminal and short, unbranched files of gonimoblast cells mature into carposporangia. Tetrasporangia are cut off laterally from surface cells and undergo basal elongation followed by lateral expansion before dividing zonately.A. subulataexhibits a combination of distinctive vegetative and reproductive features when compared withSolieriaand thus serves as the type of a new tribe, the Agardhielleae, in the family Solieriac