A study was carried out to find distribution of Legionella spp. in hot water supplying systems of buildings in Seoul. Water samples were taken from the hot water taps in three research institute buildings during the period from the 2nd to 27th February, 1987. And all of the three buildings were supplied with hot water from the same central hot water tank. GVPC BCYEa and blood agar were used for the isolation of Legionella pneurnophila and slide agglutination test (SAT) was performed to identify their strain types. Main findings were as follows 1. 44 samples taken from hot tap-water revealed 20% of positive culture and 44 samples taken from hot water spouts revealed 11% of positive culture. 2. The 78% of positive samples taken from hot water was obtained from 25.1$circ$C~45.0$circ$C temperature range. 3. Only, Legionella pneumophila serogroup 3 was identified and most of them has been than 2$mu$m in length. 4. The densities of the Legionella pneurnophila isolated from hot water samples were within the range of $1.0 imes 10^2$ to $1.1 imes 10^3$ CFU/l and their densities isolated from hot water spouts were either 1 CFU/plate or 2 CFU/plate.