Heterotrophic plate counts (HPC) and total coliforms (TC) are two major microbial indicators that are used to monitor the potability of water. Although the presence of heterotrophs has been hypothesized to predict the presence of TC, there have been few documented reports. Intensive sampling of raw, treated effluent and distribution water from a public water supply serving 400,000 people provided an opportunity to study the relationship between these two indicator groups of bacteria. A total of 26,158 samples were analyzed, including 12,970 from 1986 and 13,188 from 1985. There were 13,429 samples from the distribution system, 5,524 from treatment effluents, and 7,205 from raw water. The associations between HPC and TC were made on both a hits-and-misses and numerical comparison (CFU per milliliter) basis. The periodicity of the two indicators was also analyzed to determine whether the presence of one group could predict the presence of the other. Atypical bacteria were also related to the presence of these two indicator bacteria. Venn diagrams and nonparametric statistics revealed the following correlation coefficients for HPC and TC for 1985 and 1986 combined: raw water r = 0.45, treated effluent r = 0.06, and distribution system r = 0.10. Atypical bacteria showed a similar relationship with HPC. There was no predictive periodicity for HPC and TC within +/- 10 days of isolation of each other. Therefore, in a 2-year survey of a public water supply, the presence of HPC had a low correlation coefficient with TC, as determined by hits-and-misses and numerical comparisons. The enumeration of one group was found to be independent of the other.