Acute 96-h LC50 values of the water-accommodated fraction (WAF) of crude oil, dispersants (Corexit 9500 and Corexit 9527) and dispersed oil combinations were determined in semi-static bioassays with seawater, using the amphipod Allorchestes compressa (Dana). Sub-lethal bioassays (suppression of burying behaviour over 30 min and 24 h exposure) were also conducted for these toxicants, using the marine sand snail Polinices conicus (Lamarck) as the test organism. Sodium dodecyl sulphate (SDS) and zinc sulphate were used as reference toxicants and identical bioassays were conducted using these compounds. The mean (n = 4) 96 h LC50 (SE) values for WAF of crude oil, Corexit 9527, Corexit 9500, dispersed oil (9527) and dispersed oil (9500) were 311,000 ppm (5760), 3.03 ppm (0.05), 3.48 ppm (0.03), 16.2 ppm (2.8) and 14.8 ppm (0.8), respectively. The mean (n = 4) 30 min EC50 (SE) values were 190,000 ppm (5600), 50.2 ppm (2.1), 58.9 ppm (3.1), 65.4 ppm (1.95) and 56.3 ppm (1.9) for WAF of crude oil, Corexit 9527, Corexit 9500, dispersed oil (9527) and dispersed oil (9500), respectively. These values reduced to 43,800 ppm (1400), 33.8 ppm (0.7), 42.3 ppm (1.1), 26.3 ppm (1.3) and 24.9 ppm (1.4) after 24 h exposure for WAF of crude oil, Corexit 9527, Corexit 9500, dispersed oil (9527) and dispersed oil (9500), respectively. These LC50 and EC50 values indicated that dispersed oil combinations were significantly more toxic to these organisms than WAF of crude oil. Caution should thus be used when deciding to use chemical dispersion as a remedial action for an oil spill in temperate inshore Australian waters.