This study was performed to compare the selectivity of the fifteen-tube Standard Methods most probable number (MPN) lactose-fermentation confirming step to the sensitivity of the enzyme B-galactosidase which is the confirming factor for total coliforms in the chromogenic substrate Quanti-Tray MPN method available from Colilert. This was accomplished by running split samples utilizing both methods over a five-month period which included different levels of stratification and algal blooms in a large reservoir. A total of 73 samples were run in parallel over this period. The average MPN value using the Quanti-Tray system was 2020 while the average using the Standard Methods 15-tube MPN was 14. The highest MPN values were 17,890 with the Quanti-Tray system and 109 with the Standard Methods MPN. The average number of tubes producing gas increases from 4 to 13 when the lauryl tryptose broth tubes and the brilliant green lactose bile broth tubes were allowed an additional 120 hours of incubation in the Standard Methods MPN. This would have increased the average MPN value to 900 with gas being produced in 5, 5, and 3 tubes in the confirmation step. Isolates from tubes that produced gas in 48 hours were speciated as Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacea, and Citrobacter freundii. Isolates from tubes producing gas after the extended incubations were speciated as Enterobacter aerogenes, Enterobacter agglomerans, Enterobacter cloacae, and Enterobacter amnigenus. Isolates from tubes with growth without gas were speciated as Hafnia alvei which is also a total coliform. All isolates, that were taken from positive wells from the Quanti-Trays and were speciated, matched those total coliforms isolated from the Standard Methods MPN before and after the extended incubations. The results of this study show that the chromogenic substrate Quanti-Tray method that relies on the enzyme galactosidase as the confirming factor for total coliforms is more accurate than the lactose-fermentation confirming step for the Standard Methods MPN. The Standard Methods 15-tube MPN method does not enumerate the slow and non-lactose fermenting coliforms in the environment. Includes 14 references, tables.