Recent investigations have revealed that cryptic diversity (both phenetic and genetic) is widespread in photo-autotrophic protists. However, little is as yet not known about the taxonomic, ecological and biogeographic significanca of this phenomenon. Through the study of a number of case-studies, the current project proposal aims at answerint the following questions : 1) What phenetic and genetic, infraspecific variation patterns exist in the case-studies and how are they related to each other? 2) What taxonomic and biogeographical conclusions can be drawn from this variation analysis? 3) What is the relationship between these variation patterns and ecological factors? For culture maintenance (isolation, identification) and crossing experiments an inverted microscope is applied for.