The compartments of the Paramecium digestive system were investigated with wheat germ agglutinin (WGA). By use of cryosectioning or Lowicryl K4M embedding combined with pulse-chase studies and WGA-gold labeling, WGA binding sites were located on membranes ofthe phagosome-lysosome system, including all four stages ofdigestive vacuoles, the discoidal vesicles, acidosomes, and lysosomes. In addition, the contents of lysosomes, cisternae at the trans face of Golgi stacks, and coated and uncoated blebs and vesides at the putative rans Golgi network bind to WGA. Crystalcontaining vacuoles characteristic of mid-log to stationaryphase cultures are endosed by heavily labeled membranes. Alveoli underlying the plasma membrane sometimes contam binding sites, particularly on their outer membranes. Ciliary membranes previously shown to be labeled with WGA-FITC are negative in frozen thin and Lowicryl K4M WGA BINDING SITES OF PARAMECIUM 197