Bryozoans are common biofoulers of under-drain filter nozzles in rapid gravity filters in water treatment works. A potential method for controlling bryozoan biofouling is the use of chlorine in backwash water. Repeatedly exposing bryozoan colonies with chlorine for 20 min every 24 h in an experimental setting, to replicate what would occur if the backwash was chlorinated, caused significant reduction in colony growth and size. After 10 days repeated treatment in good conditions for bryozoan propagation, the EC50 (the chlorine concentration required to decrease growth such that treated colonies were half the size of control colonies) was 1.6 ppm (SE 0.3). In sub-optimal conditions for propagation, the impact of chlorine was greater. The majority of colonies treated with 1 ppm and above did not grow or even decreased in size over 5 days. However, a chlorine concentration of 5 ppm was necessary, even in sub-optimal conditions, to ensure all colonies decreased in size over 5 days of treatment; this is too high to be acceptable to water companies due to the risk of carcinogenic by-products. Nevertheless, the observed decline in feeding activity of bryozoans exposed to chlorine levels >1 ppm suggests that repeated backwashing with chlorine may cause colony death over time, especially in sub-optimal conditions. Chlorine backwashes may therefore be an effective long-term control strategy, especially in locations such as rapid gravity filters where it is suggested that upstream processes are likely to create sub-optimal conditions for bryozoan growth.