Chaetoceros dichaeta Ehrenb. is the type species of the genus Chaetoceros and is one of the dominant planktonic diatom species of the Southern Ocean. A number of varieties and forms have been described for C. dichaeta, which prompted us to investigate the morphological variability in clonal cultures to achieve a better circumscription of the morphological features of this species. We studied eight clonal cultures obtained from the type locality by sequencing the D1-D3 hypervariable domains of the LSU to assess their genetic identity, following cell-size reduction and sexual reproduction, and examining the morphological features of cells at different stages of the size reduction process using both LM and EM. Observation of sexual reproduction within clonal strains demonstrated that C. dichaeta is homothallic. The size and morphology of the cells vary considerably during size reduction: cell volume is reduced by over an order of magnitude, and the gradual diminution of the apical axis is accompanied by the elongation of the pervalvar axis. All forms and varieties described for C. dichaeta can be ascribed to the species, with the exception of C. dichaeta f. tenuiformis L. Mangin.