A novel microsporidian endoparasite replicating within the nucleus of Saccamoeba limax isolated from a pond Volume 19 Endocytobiosis and Cell Research 120-126 endoparasite, nucleus, Saccamoeba, spore, fine structure, microsporidia Among a mixed population of various free-living amoebae (FLA) developing from samples derived from a pond in the Luneburg Heath, individual saccamoebae were observed infected by intracellular parasites. The host cells were identified as Saccamoeba limax according to Page. Light- and electron microscopy revealed that only the nucleus of host amoebae were infested whose young stages of the endoparasites (KSL-3) had grown to large sporogonic stages followed by differentiation into round or pear-shaped spores of about 1 µm in diameter. Spores were enveloped by a double wall comprising an anchoring disc, polar tube, and polar fila-ments as characteristic traits of microsporidia. By comparing the present eukaryotic parasites with previously described eukaryotic organisms the conclusion was drawn that KSL-3 represent as yet undescribed member of microsporidia. The host strain “SI-3” was deposited at CCAP as Saccamoeba limax under the CCAP-number: 1572/5. 2009 1984 jportal_jpjournal_00000006 2009-09-09T09:51:49.373Z 2010-12-16T12:05:55.645Z