Mussel watch pollution monitoring programmes, performed at six predetermined stations on the Island of Penang, Malaysia, showed that levels of the pollutant pp′-DDE ranged between 3·7 and 17·4 ppb, while PCBs ranged between 99·9 and 599·9 ppb on a dry weight basis. Similar studies on samples from three stations in the Republic of Singapore varied from 7·11 to 8·12 and 139·8 to 256·4 ppb, respectively, for pp′-DDE and PCBs. The study further indicated that other organic chlorinated hydrocarbons were not detectable in this marine bivalve at either locality. Their absence in this instance could be a feature of the tropics, resulting perhaps from a high rate of degradation, purification or a geographical separation between the source of the pollution and the population investigated.