Abstract: A pharmaceutical dosage form for administration of medicament to fish has an outer layer of animal or vegetable material surrounding at least one inner chamber containing one or more biologically active agents. The outer layer, which is composed of animal and/or vegetable material and/or an aqueous extract from marine materials, and an optional binder, is substantially impermeable to water and the medicament. The chamber is filled with the active agent in a viscous pharmaceutical formulation to improve the bioavailability and stability of the active agent. The chamber may be partly filled with gas. The buoyancy of the dosage form in water is manipulable by adjusting the volume and contents of the inner chamber.