The prevention and control of legionellosis in water supply systems involves environmental comprehensive sampling. Our aim was to investigate occurrence of Legionella spp. in water supply systems in different administrative districts of Riga, and to find out if there exists a correlation between Legionella spp. positive cases and water source, type of sample, sampling point and temperature of hot water, received at the point of consumption. A total 467 samples were collected from 94 apartment buildings in different administrative districts of Riga. Water samples n=302) were taken from heating units in apartment buildings, from taps and shower heads in private apartments and sediment samples were taken in private apartments (n=165). Water temperature was measured during sampling. L.pneumophila was found in 50 out of 94 apartment buildings (53%). From all Legionella spp. positive samples, 19% represented L.pneumophila serogroup 1 and 81% L.pneumophila serogroup 2.-14.(15.). L.pneumophila was found in 44% of hot water samples from heating units and in 52% of hot water samples from private apartments. Data analysis confirmed observation that temperature of hot water significantly affects the frequency of L.pneumophila positive cases (p<0,05). L.pneumophila was found in 5% of cold water samples from heating units and in 17% of cold water samples from private apartments. L.pneumophila was isolated from 4% of sediment samples. However, investigation of sediment samples did not show presence of Legionella spp. in 79% of cases when at least one water sample from the same location was L.pneumophila positive