Coagulation and subsequent chlorination are typically used for surface water treatment in order to produce potable water. Chitosan, a linear cationic polymer of high molecular weight, has recently been proposed for application in drinking water treatment as potential alternative to metal species based coagulants. However, due to its nature some organic matter may be released during coagulation step, thus increasing the concentration of disinfection by-products as well as the toxicity of finished water. In the present paper the efficiency of chitosan and conventional coagulants (aluminium sulphate and ferric chloride) was compared in terms of turbidity and natural organic matter (NOM) removal, as well as acute toxicity on Daphnia magna of coagulated and coagulated/chlorinated surface water. The coagulation experiments were carried out by jar test using low TOC and high alkalinity surface water. Turbidity and NOM removal, as well as toxicity were investigated (i) under actual conditions, (ii) for different pH (5, 6, 7) and (iii) in humic acid (HA) spiked surface water samples. All coagulants decreased toxicity on D. magna from 100% to 0% immobilization. However, the addition of humic acids affected final toxicity in different ways according to coagulant type and dose. Moreover, the chlorination step after coagulation increased toxicity too according to the coagulant type as follows: chitosan > ferric chloride > aluminium sulphate. (c) 2008 Elsevier B.V. All rights reserved.