The presence and distribution of the cytoskeletal protein centrin in the lower eukaryote Giardia were determined using a monoclonal antibody to algal centrin. Immunofluorescence shows that centrin is a major component of rodlike structures associated with the flagella and strongly attached to the ventral disc and the 2 nuclei. Two groups of spots are also highlighted at the top of each nucleus where the basal bodies are situated. Western blot analysis revealed a single 23-kDa polypeptide. These findings support the ubiquity of centrin and its ancient evolutionary origin and also underline cytoskeleton complexity of Giardia, the reportedly most primitive eukaryotic organism known.