Contradictory opinions are at present held on the subject of bacterial standards of purity of water in India. My recent researches tend to throw some light on some of the causes of this divergence of opinion. The presence of frogs in water has been found to be responsible for an increase in faecal bacilli which, though negligible in the case of large bodies of water, is apt to be misleading in the case of waters of great purity. Stasis in reservoirs and passage through pipes may be responsible for a considerable increase in faecal bacilli in water, so that a mere increase in the number of faecal bacilli in a reservoir or pipe water does not necessarily indicate faecal contamination but may simply be due to stasis in reservoirs or flow through pipes. These findings lead to two conclusions of importance in regard to the bacteriological examination of water: (1) in testing for the bacterial efficiency of sand filters, samples should be collected from the filter well and not from the clear water reservoir where water is usually stagnant for some hours; (2) in interpreting the results of the examination of pipe waters the possibility of an increase of B. coli having occurred through multiplication in the pipes should always be borne in mind before inferring that such increase is indicative of faecal contamination or leakage in the mains.