Morphotaxonomy, ecological preferences and biological characterization of algal taxa of order Zygnematales (Conjugatophyceae, Chlorophyta), mainly the freshwater species Spirogyra, Zygnema, and Mougeotia are still poorly understood and need further in-depth investigations. In this study, different Spirogyra strains were examined to characterize their abilities in conjugation and formation of zygospores under different environmental conditions. We found that 16:8-h light:dark photoperiod is the best condition to induce the conjugation and zygospore formation. Moreover, the sexual reproduction was noticed to be triggered with increasing the light intensity to 85 μmol photons m-2s-1; no conjugation was observed at 35 μmol photons m-2s-1. The attribute of conjugation under red and blue irradiance, respectively, never equalized that in the white light even at elevated intensity. pH value (7.5) was the most suitable niche for induction of sexual reproduction. An increase of CO2 in the atmosphere, provided by NaHCO3 solution, did not enhance sexual reproduction. Cultivation of the investigated Spirogyra strains on 0.003% CaCl2-containing agarized Pringhsheim´s medium (1/2 conc. and without nitrogen) induced the conjugation process as in case of CaCl2 omitted. UV radiation completely inhibited the conjugation at all growth conditions.