INTRODUCTION On 24 September 1962 a number of Tardigrada was obtained from a sample of lichens from the French Alps. The lichens had been collected Figs. 1, 2. Pyxidium tardigradum n. sp. on Hypsibius oberhaeuseri (Doy.). on 27 June 1961 from a rock in the department Haute Savoie and had been preserved in a dry condition for nearly fifteen months. After washing, living specimens of three different species of Tardigrada (Hypsibius oberhaeuseri (Doyère), Macrobiotus hufelandii Schultze and Echiniscus spec. were found, together with some eggs of Hypsibius oberhaeuseri and Macrobiotus hufelandii, a small number of Rotatoria and some dead Oribatid Acari. About twenty Protozoa appeared to be attached to one of the specimens of Hypsibius oberhaeuseri (fig. 1). It was thought of interest to study these protozoan symphorionts. OBSERVATIONS At first the identity of the epizoa could not be determined as they were contracted and in an inactive state. Only nuclei were visible in the cells. The Tardigrade to which these Protozoa were attached, at first was alive; the foremost part of its body was contracted, but the legs moved. The animal was kept in the cavity of a slide partly covered by a cover-glass. The cover-glass was glued to the slide so that it could be placed vertically in a glass-tube with water. This is an easy way to keep such open slides for several days, while the object can be studied regularly, even with oil immersion. After one day the Tardigrade did not move any more but in one of the Protozoa a pulsating contractile-vacuole could be seen. A day later contractile-vacuoles were visible in three or four specimens and two specimens