The ultrastructure of Kupffer cells of canine liver has been studied and described. Kupffer cell nuclei were elongated and irregular in shape having a prominent double nuclear envelope containing scattered ribosomes on the outer membrane. Mitochondria were small, relatively scarce, and contained few mitochondrial granules. Elements of endoplasmic reticulum were scarce and highly irregular in form. In general, the cytoplasm was less dense than hepatocytes and contained fewer organelles which were less compactly arranged than in hepatocytes. Microbodies were not observed in any of the Kupffer cells studied. The cytoplasm contained various numbers of large phagosomes but consistently contained numerous dense cytosomes of various sizes. The most striking finding was that many cytosomes and phagosomes contained large numbers of short microtubules approximately 350 Å in diameter which were arranged in clusters and were associated with membanes.