Sediment samples were collected from different sites including Tolo Harbour, Victoria Harbour, Deep Bay and the south of Lantau Island (a reference site) to determine the existing impacts of sediment-associated contaminants on fish. Many biomonitors have been used to study water pollution in Hong Kong, however, the use of molecular studies on the sublethal physiological effects of coastal sediment exposure in fish is limited. The use of tilapia to detect cytochrome P4501A1 (CYP1A1) and metallothionein (MT) gene expression in liver tissues using RT-PCR was studied in the present project. The fish were exposed for 7 days to coastal sediments. Three control groups were also used; one 'negative' control group was maintained in seawater only, while the second and third 'positive' control groups were injected i.p. with β-napthoflavone (40 μg/g body weight) and cadmium chloride (10 μg/g body weight), respectively. The results showed that exposure of tilapia to the sediments induced MT and/or CYP1A1 levels in the liver tissues. This enables us to devise highly sensitive biomarkers for monitoring metal and trace organic contamination in local waters. Copyright © 2000 Elsevier Science Ltd. All rights reserved.