定鞭藻门是一类原生生物。属于囊泡藻界。1955年Parke等引入了"定鞭"(Haptonema)来描述细胞顶端的第3根鞭毛。其主要特征还有细胞表面包裹鳞片。

The haptophytes, classified either as the Prymnesiophyta (named for Prymnesium) or Haptophyta, are a division of algae.

The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at the class rank rather than as a division. Although the phylogenetics of this group has become much better understood in recent years, there remains some dispute over which rank is most appropriate.