The connective tissues of a wide range of invertebrates have been investigated using a potassium permanganate/spirit blue staining technique. The method demonstrates fibres which are distinct from collagen, vertebrate elastic fibres, and reticular fibres. The fibres are variously associated with subepidermal collagen, muscles, basement membrances, the walls of blood vessels and the sheaths around nerve cords; the method also gives a positive reaction with the arborescent mesogloeal fibres of medusoid coelenterates. Their electron microscope appearance is distinctive and they exhibit a physical elasticity. Chemical tests indicate that following pretreatment with acidified potassium permanganate they show many, although not all, of the properties of vertebrate “elastin”. It is concluded that they should be regarded as a special category of “elastic” fibre.