Abstract: The harvesting of the products in the mass culture of unicellular green algae is one of the most important procedures in their cultivation,but its problems are not yet completely solved. In the mass culture of Scenedesrnus obliquus we have tried·HC1,KOH,NaOH,Al2（SO4）3; CaO,and saturated lime-water（Ca（OH）2）as agents for precipitating the algae in cultures.Among these,HCl,KOH and NaOH are not very effective,and after treatment the algae become yellowish and precipitate very slowly.On the other hand,treatment with 0.03—0.05% of alum,or with 6.0% of saturated lime-water,results in the complete precipitation of the algae within 2 hours.It is, however,of considerable interest,that if we add lime directly to the culture medium,no or very little effect is observed.Also,the use of vessels of different shapes for when carrying out these treatments with chemicals,shows some influence on the rate of precipitation of the algae. Although the mechanisms involved in the precipitation of cultivated algae by saturated lime-water are not completely known,it remains certain that this method is an effective one,and is of practical value in the mass culture of algae and their more efficient harvesting.