The spore-forming bacteria usually found in water are those of the subtilis group, derived largely from soil washings, and _B. enteritidis sporogenes_, from sewage and manure. The spores of these organisms are very resistant and survive all ordinary concentrations. Wesbrook et al.[1] found that 3 p.p.m. of available chlorine had little effect on a spore-forming bacillus isolated from the Mississippi water and the author has obtained similar results with _B. subtilis_. Thomas,[2] during the chlorination of the Bethlehem, Pa., supply, found four organisms that survived a concentration of 2 p.p.m. of available chlorine: _Bact. ærophilum_, _B. cuticularis_, and _B. subtilis_, all spore formers and _M. agilis_. In practice no attempt is made, except in special cases, to destroy the spore-bearing organisms as they have no sanitary significance and the concentration of chlorine required for their destruction would cause complaints as to tastes and odours if the excess of chlorine were not removed. Such doses are unnecessary and result in waste of material. It is found that, when the dose is sufficient to eliminate the _B. coli_ group from 25-50 c.cms. of water, the majority of the residual bacteria are of the spore-bearing type. Smeeton[3] has investigated the bacteria surviving in the Croton supply of New York City after treatment with 0.5 p.p.m. of available chlorine as bleach. Table XVII gives the results obtained. The organisms of the _B. subtilis_ group outnumbered all the others, 66 (62.8 per cent) belonging to this group alone. This group contained _B. subtilis_--Cohn (36 strains), _B. tumescens_--Chester (15 strains) _B. ruminatus_--Chester (13 strains), and _B. simplex_--Chester 1904, (2 strains). Three of the four coccus forms were classified as _M. luteus_. No intestinal forms were found. Clark and De Gage[4] in 1910 directed attention to the fact that the bacterial counts, made at 37° C. on chlorinated samples, were often much greater than the counts obtained at room temperature. "This phenomenon of reversed ratios between counts at the two temperatures," they stated, "has been occasionally observed with natural water, but a study of the record of many thousands of samples shows that the percentage of such samples is very small, not over 3-5 per cent.... On the other hand 20-25 per cent. of samples treated with calcium hypochlorite show higher counts at body temperature than at room temperature." Clark and De Gage were unable to state the true significance of this phenomenon but were of the opinion that it was not due to larger percentages of spore-forming bacteria in the treated samples. Other observers, on the contrary, have invariably found the spore-formers to be more resistant to chlorine and thermophylic in type. TABLE XVII.--ORGANISMS SURVIVING TREATMENT NEW YORK (SMEETON)