The efficiency of chloroisocyanurates in swimming pool disinfection has been demonstrated by a number of authors (Andersen, 1965; Kolwalski and Hilton, 1966; Robinton and Mood, 1967; Fitzgerald and der Vartanian, 1969). The disinfection rate was seen to slow down during studies on different bacteria; this was attributed to the existence of a weak concentration of free chlorine (HOCl + ClO-). However, since these studies were carried out when the aqueous chemistry of chloroisocyanurates was not completely elucidated, the specific action of chloroisocyanurates was difficult to determine. This, in part, was due to the large number and variety of experimental procedures and analytical methods for chlorine measurements, and to the lack of specificity of the methods employed. We studied the action of chloroisocyanurates on three environmental strains: S. faecalis, S. aureus, Ps. aeruginosa and on one collection strain: Ps. aeruginosa-CIP. Disinfection tests were carried out in the laboratory, in pH and temperature conditions typically found in swimming pools (pH 7.5, 25°C). The initial concentration of total chlorine varies from 0.5 to 2.5 mg I-1, for concentrations of isocyanuric acid, H3Cy, from 25 to 360 mg 1-1. With these concentrations, free chlorine (HOCl + ClO-) does not exceed 3% of total chlorine (that is, less than 0.08 mg 1-1). We used an easy and rapid method of dosage which distinguishes between "available chlorine" [(HOCl + ClO-) + (chlorine from isocyanurates, HnClmCyp-)] and "total chlorine": absorption spectrophotometry after reaction with the N-diethylparaphenylenediamine (DPD). We adopted a common method for monitoring simultaneously, chlorine concentration and bacterial count. In this way, we studied the kinetics of selected bacteria inactivation, relative to H3Cy concentration and for different levels of available chlorine