1. Lochmiopsis, a green alga belonging to the family Chaetophoraceae was described by Woronichin in 1927 from lakes in the Kulundin-steppe, west-Siberia. This alga was probably already seen by Loew in 1877 in Owen’s Lake, California and was rediscovered by G. M. Smith and L. G. M. Baas Becking in Marina, California in 1927 and in Mono Lake and other places in California in 1929. 2. The alga is polymorphous which accounts for the contention of Woronichin that two forms, Lochmiopsis sibirica an L. Printzii do occur. 3. The alga is halophilic in concentrations of NaCl lower than 1.5 mol.; the resting stages are halotolerant up to a saturated salt-solution. 4. pH, temperature and salt-influence were studied. They show that the alga is adapted to carbonate and chlorocarbonate-waters of the temperate zone. Occurrence of the alga may be predicted in Europe, South-Africa and South-Australia from the data presented in this paper. 5. The natural environment lies well within the potential milieu.