A Study on environmental sanitation on the well in Seoul was made to find the degree of Physio-chemical and Bacterial contamination. For this study 30 wells were located at Nokbundong Hongjaidong and Hongeundong from September 13 to 22 1971. As the results of this study the following conclusions were obtained. 1) Seventy per cent were being utilizing by from 10 to 50 households per well. 2) About 93 per cent of the wells were found to be dug out in Rocks. 3) Eighty-three per cent of the wells surveyed in the slums had a depth of water shorter than 20 cm. 4) The degree of Total Viable Bacteria contamination were high point $15{ imes}10^2$ low point 13 and coliform bacteria were high point $79{ imes}10^1$ low point 1 organism per ml. 5) Only 4 wells (13.3%) were able to drink but 26 wells (86.7%) were not drinking by physio-chemical and Bacterial contamination.