Aggregation factors, purified from 14 sponges which belong to the classes Tetraxonida and Cornacuspongia, were visualized electron microscopically. Two types of basic structural forms were detected; first, circular structures from the species Ancorina cerebrum, Mycale massa, Hemimycale columella, Crella rosea, Clathria coralloides, Axinella cannabina, Pellina semitubulosa, Ircinia muscarum, Hippospongia communis, Verongia aerophoba and second, rod-like structures from Tethya lyncurium, Tedania anhelans, Hymeniacidon sanguinea, Dysidea tupha. In most of the cases the structures carry side chains.