The occurrence of cyanobacterial blooming is typical for the Lake Mazais Baltezers, which is the water source for artificial groundwater recharge plant supplying to 25% of drinking water for Riga City. The investigations of algae and bacteria were carried out in 1997-98 in the lake as well as in water, sediment and vadose zone of infiltration basin. The blooms of Cyanobacteria appear in water of the lake and infiltration pond in July (30400 and 2768400 cells l^-1 or 0.381 and 0.847 mg l^-1 correspondingly) when Microcystis spp. is dominating, and in August (888200 and 17997300 cells l^-1 or 0.847 and 1.925 mg l^-1 correspondingly) when the mass blooming of Aphanizomenon flos-aquae is stated. The development of algae as well as bacteria (psychrophillous, mesophillous and oligocarbophillous aerobic and facultatively anaerobic saprophytes, total bacterial number) in water of infiltration basin is substantially higher. Cells of Cyanobacteria are observed to the depth of 3cm under the contact layer ìwater-sedimentî in infiltration basin, and in contrast to water, Cyanobacteria in sediment are detected not only in the late summer, but also in other seasons. The credible correlations between the cell numbers of Cyanobacteria and saprophytic bacteria of different groups, and total bacterial number are not established in water as well as in sediment. The investigations of 10 bore-holes (on the way from infiltration pond to siphon-pipe) with the depth to 900cm has shown that Cyanobacteria develop only in the upper layer of infiltration pond while bacteria are stated up to the depth of 900cm in all bore-holes.