Observations of the behaviour of the common cockle,Cerastoderma edule(L.), exposed to various concentrations of industrial Chloros (sodium hypochlorite; NaOCl) in the laboratory for 24 h, were made prior to, during and after exposure. Five days after removal of Chloros no abnormal behavioural responses were observed at concentrations below 15 ppm. Between 15–25 ppm NaOCl behavioural changes were observed; cockles showed foot protrusions, shell value gaping and suffered low mortalities, whereas at concentrations above 25 ppm NaOCl there was 100% mortality. Cockles, like many species of bivalve, contain within their shells a complete record of the environmental conditions to which they have been exposed throughout their life. The record takes the form of tidal microgrowth bands which are laid down in the shell structure. Marked cockles grown for 14 d in a simulated laboratory tidal regime of 9 h immersion : 3 h emersion were exposed to Chloros for 9 h (1 simulated high tide). One group was then allowed to grow for a further 7.5 d and another group for 22.5 d. Acetate peel replicas of the sectioned shells showed that all cockles subjected to Chloros produced a cleft in the shell as a result of shell valve closure and withdrawal of the mantle edge during exposure. Post‐Chloros shell growth was disrupted with the deposition of deformed growth increments; normal incremental growth only resuming after several days. Control cockles showed none of these features. Shell valve movements of cockles prior to, during and after exposure to Chloros were recorded. Before exposure to Chloros a regular pattern of shell valve opening and closing was observed, corresponding to high and low tides respectively. During an 8 h exposure to Chloros immediate shell valve closure occurred with subsequent erratic shell valve activity. Upon immersion in fresh sea water the normal pattern of activity was immediately resumed, although the amplitude of shell gape was reduced.