Three species of tropical estuarine invertebrates were exposed to copper sulfate and cadmium chloride to investigate their potential as test specimens for sediment toxicity assays in the South-east Asian regions. The larvae of the reef sea urchin (Diadema setosum), the oyster (Crassostrea iradalei), and the mud crab (Scylla seratta Forskall) were used in the 48-hr assays with copper and cadmium as reference toxicants. In addition the sea urchin were tested for end point measurements at different stages of the larval development and a 60-min sperm bioassay. The study revealed that the sea urchin first cleavage, which is an assay end point and which takes place about 1 hr after fertilization, was the most sensitive stage for both toxicants, with copper being more toxic than cadmium. Sensitivity comparisons between the three invertebrate larvae revealed the mud crab zoea larvae to be most sensitive for cadmium with an LC50 value of 0.078 microgram/ml, while the sea urchin was more sensitive for copper, with EC50 values of 0.01 microgram/ml at the first cleavage stage and 0.04 microgram/ml at the pluteus larva stage. All the invertebrates tested gave responses that made them suitable test organisms for metal bioassays in the tropical estuarine environment.