Abstract
Thalassiosira catharinensis is a new nanoplanktonic diatom 4.4–6.6 µm diameter. The diagnostic characteristics are the presence of fultoportulae and rimoportulae with short external tubes of the same diameter, rimoportula located at same level as fultoportulae, the presence of three satellite pores around marginal and central fultoportulae, and rimportula with small, shallow internal aperture. The new species is compared to similar taxa such as Thalassiosira laevis, T. mala, T. minima, T. profunda and T. pseudonana.