A cDNA clone encoding a putative Bop1 homologous protein was identified in Giardia lamblia. Since Bop1 is a nucleolar protein involved in rRNA processing, thereby controlling the cell cycle, we investigated components of cell cycle control in G. lamblia by identifying the protein(s) that interact with Bop 1. Through an immunoaffinity column made with polyclonal antibodies specific to the recombinant Bop1 of G. lamblia, a pool of proteins was obtained from the crude extracts of Giardia and then used as antigens to immunize rats. By employing the resultant sera for cDNA library immunoscreening, we isolated cDNA clones encoding an immunopurified protein, which turned out to contain the gene for beta-giardin, a Giardia-specific cytoskeletal protein. The interaction between Bop1 and beta-giardin was confirmed via two different methods, yeast two-hybrid assay and coimmunoprecipitation.