An argillaceous, monothalamous foraminifera Tolypammina vagans (Brady) adapted to live in sponges, was found in the water system of numerous siliceous sponges collected from Oxfordian marls and limestones of Central and Southern Poland. Individual variability of T. vagans was traced and it was found that most species of Tolypammina Rhumbler should be classified within this species. The occurrence of T. vagans in sponges was commensal in character and dependend on: 1) dynamics of water circulation in sponges, 2) presence of sufficient quantity of terrigenous quartz in silt fraction in water. The role of occurrence of T. vagans in successively changing sponge assemblages for the reconstruction rate of silt sedimentation in the Oxfordian Basin of Poland is pointed out.