This paper presents results obtained in a pilot scale investigation of biological treatment for preparation of drinking water from the North Saskatchewan River at Edmonton, Canada. Although the concentration of natural organic matter (NOM) in the raw water varied substantially over the study period, parameters measuring the biodegradability and reactivity to chlorine of the NOM were closely correlated to the NOM concentration. As a result of treatment including ozonation and adsorption, two patterns of response emerged. Some parameters decreased through each step of the treatment process while others usually increased following ozonation and then decreased through subsequent steps. The levels of this latter group were decreased by biological activity in the filters (except for one treatment stream) and further reduced in the biologically active granular activated carbon (GAC) contactors.