The levels of trihalomethanes (THMs)--the main species of by-product from water chlorination--were monitored in the distribution systems of the five major drinking water utilities of the greater area of Québec City in order to investigate and model their occurrence on a spatial and seasonal basis. Data for THMs and other water quality and operational parameters associated with their formation were generated through a 16 month sampling program involving several sites representing variable water residence times, from the plant to the system extremity. The results demonstrate that the differences in measured THM levels between the five utilities are mainly due to the variable quality of raw waters, the type of water treatment process being used and the type and levels of applied disinfectant. Depending on the utility, average THM levels were from 1.3 to 2.5 times higher in the system extremities than in the water leaving the treatment plant. Also, average levels of THMs measured in summer at the distribution system extremities were, depending on the utility, from 2.5 to 5 times higher than the average levels measured in winter. The seasonal differences were found to be significantly greater than those observed by others in water utilities in the United States and Europe and are explained in large part by the considerable changes, over the year, in the quality and temperature of surface waters in Southern Quebec. For the live utilities under study, multivariate regression models were developed in order to predict spatial and seasonal variations of THMs. Both residual chlorine demand and temperature were found to be better, statistically, as predictors for THM occurrence. The usefulness of the developed models for routine and long term water quality management, as well as for assessment of human exposure to THMs, are also discussed.