Natural coliform populations in finished drinking water samples held at 5 and 22C declined significantly after 24, 30 and 48 h as measured by the membrane filter (MF) and multiple tube fermentation, most probable number (MPN) methods. Twelve samples held for 24 h at 5C and 22C lost 34% and 87% respectively, of the intial MF count. Initial unacceptable counts ranging from 31 to 5/100 mL for 5 samples (42%) declined to acceptable levels of 4 or less/100 mL by 24 h. The most significant effect on coliform rates of decline came from holding temperature. The rate of decline was 4.5 times greater at 22C than at 5C. Regression analysis indicated that for samples held at 5C the rate of decline increased with high pH values and increased progressively over time, measured in days from the start of the study. This study indicated that samples with initial coliform counts as high as 100/100 mL could have been reported as satisfactory. For the waters examined, a holding time of 30 h at ambient laboratory temperature, as currently specified in drinking water regulations, would have resulted in inaccurate and misleading information on the coliform counts.