The oyster embryo bioassay was initially developed by Woelke (1972). The Ministry of Agriculture, Fisheries and Food (MAFF) in the UK have modified the method to improve the accuracy of the test and allow its use on board research vessels. This paper describes the modified method which has been used to obtain a measure of the deterioration in biological water quality in UK coastal' areas receiving anthropogenic discharges. MAFF have successfully used this protocol since 1976 (see Lloyd and Thain, 1981; HMSO, 1982; Thain and Watts, 1984; Byrne et al., 1985; Utting and Helm, 1985; Byrne et al., 1986; Law et al., 1986; Byrne et al., 1988). The phrase "deterioration in biological water quality" implies that a change in chemical, physical, and/or biological composition has occurred which is potentially harmful to aquatic organisms. A bioassay to measure such a deterioration should be based on a response by an organism which clearly represents a harmful effect at both the individual and the population level of organization. The lowest level at which such responses can be measured with certainty are the three 'scopes' for activity, growth, and reproduction. The organism response used in this bioassay is the ability of the oyster embryo to develop normally and reach the 'D'-shaped larval stage (at which the paired hinged shells can be seen) within 24 hours. Although the exposure time is short, it encompasses a period of intense cellular activity during which the impairment of a number of critical physiological and biochemical processes may result in poor growth and development. The response measured is, therefore, similar to that used in other early life stage tests which record growth and development, and it has the advantage that exogenous feeding is not required, thus eliminating this source of variation in the test results.