A new concept of substrate loading of activated sludge, “contact loading” is introduced, which means the instantaneous contact ratio of substrate to sludge. Using a substrate made up of glucose and peptone the activated sludges were cultivated under several contact loadings. The influence of contact loading on the capacity of polysaccharide storage and settleability related to the growth of filamentous bacteria was examined. The competition of filamentous and non-filamentous bacteria was analyzed under the different conditions of contact loading and aeration time.