AbstractThe infraspecific classification of holoplanktonic animals is treated in different ways by several authors. In this paper an attempt is made to give a general survey of our knowledge concerning the different influences responsible for speciation in the open ocean and of the kinds of speciation observed so far in holoplanktonic groups. Studies on Heteropoda, Pteropoda, Chaetognatha, Crustacea and Tunicata constitute the chief basis for the discussions. Attention is given to the species, subspecies, semispecies and the superspecies, and the concept of forma is defined. This concept forma is discussed in particular because it proves to be rather important for the classification of holoplankton which, in general, shows a slow speciation rate. The main objective of this paper is to underline the problems of speciation in holoplankton where basic differences in speciation are found in contrast to other groups. Certainly not all problems concerning these subjects are discussed, and neither is it possible to give more than a few examples of marine speciation.