During the years 1999-2003 our research was focused to the studies of molecular mechanisms of adaptation of halophilic black yeasts to extreme environmental salinities and to the studies of steroid metabolism in eucaryotic microorganisms Cellular responses to high environmental salinities were performed on halophilic and halotolerant black yeasts which were isolated as the first representatives of fungi from solar salterns in Sečovlje, Slovenia. Horatea werneckii with the capability of growth at the broad range of salinities (0-32% NaCl) was selected as a model organism to study mechanisms of adaptation to high salt concentration. We found that halophilic black yeasts respond to high salt with changes in membrane structure and properties: membranes of halophilic black yeasts are more fluid when compared to plasma membrane of yeasts, which are not adapted to high salt concentration. We identified 10 genes which are differentially expresed at different salinity in halophilic Horatea werneckii. Phisiological role in adaptation of some of them was proposed. By izolation of S.cerevisiae homologues of two key kinases of HOG signalling pathway HwHog1 and HwPbs2, we prooved similar signalling pathway which senses and responds to increased salinity, exists in H.werneckii. Identification of halotolerance genes in eukaryotic microorganisms and heterologues expression of such genes in different organisms could be of importance in development of engineered crop plants. Metabolism and role of steroids in microorganisms were studied on 17beta-hydoxysteroid dehidrogenases (17b-HSD) in filamentous fungi and mesophilic and halophilic yeasts. We isolated and characerized this enzyme at the gene and protein level from filamentous fungi C.lunatus in P.ostreatus.Three-dimensional structure of 17b-HSDcl was solved. With structural studies, the role of individual amino acids in enzyme function was determed. Knowledge of the structure and function of 17b-HSDcl as the model enzyme could help in understanding other proteins from the family of short-chain dehydrogenases/reductases.