Levels of chemical pollutants in the environment often display wide seasonal variation in response to climatic and other factors. Use of bioindicators such as enzyme activities in biomonitoring studies is complicated by this variation. Many such enzyme activities themselves show considerable seasonal fluctuation and there is known to be seasonality also in natural exposure to oxidative stress. This review attempts to explore some consequences of seasonal variation for biomonitoring studies with bivalve molluscs. It is suggested that independence of seasonal variation should be seen as a desirable feature of a bioindicator molecule. Where such molecules show seasonal variation, however, this should be incorporated into interpretation of biomonitoring studies by the use of appropriate controls.