Abstract—The relative photoinduced toxicity of five benzothiadiazine diuretics was assessed with respect to their potentiality to inactivate bacteriophage lambda by exposure to near‐UV light in a semi‐aqueous solution of 40% N,N‐dimethylformamide. The results showed the following order for the toxicity on a quantum yield basis: penflutizide>trichlormethiazide>methichlothiazide>benzylhydrochlorothiazide>hydrochlorothiazide. Since the bacteriophage was inactivated even when mixed with preirradiated solutions of the diuretics, some photoproducts are responsible for the photoinduced toxicity of the diuretics. Although a lowered pH affected the infectivity, it was a minor