Bolidophyceae is a class of photosynthetic heterokont picophytoplankton, and consist of less than 20 known species. They are distinguished by the angle of flagellar insertion and swimming patterns as well as recent molecular analyses. Bolidophyceae is the sister taxon to the diatoms (Bacillariophyceae). They lack the characteristic theca of the diatoms, and have been proposed as an intermediate group between the diatoms and all other heterokonts.