Six terpenoids and two steroids have been isolated from three marine sponges as antifoulants, viz. manoalide (1), seco-manoalide (2), manoalide 25-acetate (3), (4E,6E)-dehydromanoalide (4) from Smenospongia sp., (+)-curcuphenol (5) and (+)-curcudiol (6) from Myrmekioderma dendyi, and tri-2-aminoimidazolium halistanol sulfate (7) and halistanol sulfate (8) from Topsentia sp. These compounds showed antifouling activity but not toxicity against cypris larvae of the barnacle Balamis amphitrite, with ECS0 values of 0.24, 0.80, 0.53, 2.7, 2.5, 2.8, 4.0, and 2.9 μgml-1 respectively.