Biological nitrogen fixation (BNF) by blue-green algae (cyanobacteria) is a process of prime importance, both for terrestrial and aquatic ecosystems. Thanks to their nitrogen-fixing ability, the algae are used widely as bio-fertilizers, improving as well the soil biota, humidity, physical structure etc. An important aspect of the practical application of nitrogen-fixing algae as biofertilizers is the correct selection of species. The selection based on the taxonomic position of the species represents the most significant criterion in this sense. Related species have greater opportunities to develop polyculture and to adapt to soil conditions. Thus, the first step is the combining of species of algae, selected as bio-fertilizers during the cultivation in liquid nutrient media under laboratory conditions. The purpose of our research was to study the BNF, under laboratory conditions, by combined algal culture of different species of the genus Nostoc. The experiments were carried out under laboratory conditions for 15 days, at 27–290C and continuous illumination (120 watts/0,64 m2). The algae were grown in liquid nutrient medium Fogg, previously sterilized using UV lamp. The three equal combination of nitrogen-fixing species of Nostoc, was inoculated in the amount of 0,03 g/l, recalculated to absolutely dry biomass. BNF process was evaluated by determining NH4+, NO3-, NO2-, total N in algal biomass, fixed atmospheric nitrogen, total nitrogen and the nitrogen eliminated in the nutrient medium. Statistical significance of the means was evaluated using computer program “Microsoft Office Excel 2013”, by determining the arithmetic mean (X) and standard deviation (x). The obtained results revealed that the concentrations of nitrate and ammonium ions in nutrient medium had an increasing trend up to a certain limit, then decreased. This indicates that algae fix the nitrogen in nutrient medium to an upper limit, followed by subsequent consumption and the process is repeated continuously. The same finding was relevant for the amount of total nitrogen in algal biomass that grew up to a maximum of 342±28,5 mg/g, followed by the reduction. Fixed nitrogen concentration shown the same upward trend followed by reduction. The algae of genus Nostoc demonstrated the ability to fix 119,2±0,5 mg/l of nitrogen, 26% – 29,95% of which was eliminated in nutritive medium. These researches highlight that the BNF performed by combinations of algae of the genus Nostoc is a self-regulating process. The present findings call for future studies on combined culture of Nostoc species to exploit them in field conditions.