The objective of this study was to evaluate the influencing factors in the occurrence of coliforms in the drinking water in La Plata (Argentina) from June 1999 to June 2001. A total of 180 samples were collected from Rio de La Plata (102 samples) and Puelche Aquifer (78 samples); 45 samples were collected for each of the four seasons. The membrane filter procedure was used for isolating bacteria, and each sample was tested for chlorine and pH. The highest percentage of samples positive for coliforms in the two tested media was obtained in summer while the highest percentage of negative samples was obtained in winter. No Escherichia coli was isolated. The percentage of injured coliforms fluctuated between 70 and 100%. The most frequently isolated bacteria was Enterobacter cloacae in summer, Enterobacter agglomerans in autumn and Klebsiella oxytoca in winter and spring. Significant correlations were observed between coliforms and the distance from the initial treatment point, and with the level of free chlorine. We conclude that drinking water contamination in La Plata occurs in the distribution system due to increased temperatures and reduced disinfectant levels, which result in bacterial regrowth.