Bacterial activity at the bottom of back-filled pipe line ditches was compared with that found in normal undisturbed soil. Topics discussed include the effect on bacterial development of: availability of food materials in soil, water availability in soils, effect of oxygen content of soil, soil acidity, temperature variations, application of cathodic protection, compaction and settling of back-fill. A study was made of the role bacteria play in the deterioration of pipe line coatings. The significance of high moisture in soil adjacent to the lines, the moisture content of coatings and the presence of aerobic bacteria near the pipe lines was discussed.