Disjunct populations of vegetative and resting stages of the bipolar diatom Thalassiosira antarctica Comber were examined for differences in protoplast ultrastructure. Analysis of clonal cultures from northern and southern populations revealed differences of uncertain taxonomic value between resting spores and no discernable differences between vegetative cells. Several unique ultrastructural features were noted in members of both populations. Most notable were complex corelike structures and intramembrane particle arrays, not previously observed in diatoms, occurring in certain vesicles. Also, dictyosomes exhibited an apparent reversal in polarity with respect to width and staining intensity of luminal spaces. These and other ultrastructural characteristics are considered in relation to protoplasts of other Thalassiosira species and diatoms in general.