Mutant strains of unicellular green algae--Chlorella and Chlamydomonas which differ in their radioresistance have been investigated at different levels: cellular, molecular, biochemical and ultrastructural. It is obtained that several futures are typical for investigated strains. There are several features typical for radioresistant mutant strains investigated by us: in some cases a relationship was found between radioresistance and ssb-DNA repair efficiency; a high level of SH-groups; SOD; pigments content, especially carotenoids and chlorophyll a; a lower level of Pro content; a stability of the ultrastructural cell components or/and of the presence and structure of the cell envelope.