Abstract: A method of controlling pathogenic organisms, fungi and algae in water which includes the steps of applying to the water a primary reagent comprising a linear polymeric biguanide or a salt thereof which in its free base form has a recurring polymer unit represented by the formula ##STR1## wherein X and Y represent bridging groups in which together the total number of carbon atoms directly interposed between the pairs of nitrogen atoms linked by X and Y is more than 9 and less than 17, and a back-up algicidal agent comprising a substance which in the aqueous phase produces hydrogen peroxide, preferably persalts selected from perborates, percarbonates, peroxides and persulphates.