When coal tar is transported by sea it is categorized under International Maritime Organisation (IMO) guidelines for noxious liquids, by toxicity as well as other properties. Previous toxicity assessments are available for freshwater animals only, however, and are misleading because they were reported as nominal measures of the amount of coal tar added to the dilution water. Not all of this coal tar will enter solution, and if the data are corrected to measured concentrations of total organic carbon in solution, the true LC50 values are an order of magnitude less. Even lower values may be obtained by exposing the animals to whole coal tar in addition to the water-soluble fraction (WSF), but the concentration of whole coal tar in water cannot be measured accurately. It is recommended that such toxicity assessments should rely on measured values for WSF alone, and marine test values are reported here for the tiger prawn Penaeus monodon. The WSF was analysed by gas chromatography-mass spectrometry (GC/MS) to obtain 96-h LC50 values of 3·1 mg/litre total organics and 2·5 mg/litre total organic carbon.