SUMMARYDiatom systematics depends almost entirely upon structure of the silica shell. It is not known to what extent the taxonomic species, as defined by shell structure, corresponds to the genetic species—i.e., to the reproductively isolated population. As an approach to this problem, we report here a comparison of enzymes by electrophoresis. We have examined the genetic constitution of a number of clones of (presumably) the same species for each of 2 closely related, centric diatom species:Thalassiosira pseudonanaHasle and Heimdal andT. fluviatilisHustedt. The 4 clones ofT. fluviatilisform a distinct group, clearly separated from all theT. pseudonanaclones. WithinT. pseudonana, 4 estuarine clones and one reef clone form a group that is distinctly different from 4 oceanic clones. A single clone ofT. pseudonanafrom the Continental Slope waters is intermediate between these 2 groups and probably shares genes with both groups, indicating that the 2T. pseudonanagroups are not genetically isolated. We conclude that i) within groups, isolates are closely related even though they originated from different continents; and, ii)T. pseudonanais subdivided into ecological race