We have localized a structure-bound fraction of the exocytosis-sensitive phosphoprotein, PP63/parafusin (PP63/pf), in Paramecium cells by widely different methods. We combined cell fractionation, western blots, as well as light and electron microscopy (pre-and postembedding immunolabeling), applying antibodies against the recombinant protein. PP63/pf is considerably enriched in certain cortical structures, notably the outlines of regular surface fields (kinetids), docking sites of secretory organelles (trichocysts) and the membranes of subplasmalemmal Ca 2+ -stores (alveolar sacs). From our localization studies we tentatively derive several potential functions for PP63/pf, including cell surface structuring, assembly of exocytosis sites, and/or Ca 2+ homeostasis.& b d y :