Despite that the "centric" marine planktonic diatom Actinoptychus senarius is relatively common and appears in coasts from temperate and tropical areas all around the world, this species has not been studied in detail.Cultured material of the species,from the Mexican Pacific, was used to do detailed studies on its morphology,including observations on chain or colony formation, vegetative reproduction,auxosporulation (apparently with no spermatogenesis) and initial valves and cells.