The dsRNA virus which infects some strains of Giardia lamblia has been purified and characterized with respect to its effect on growth of the parasite. Extensive purification of the virus from G. lamblia growth medium was accomplished by Millipore filtration and two successive CsCl gradient centrifugations. The purified virus possessed a single major protein species of 100,000 molecular weight. Effects of the extensively purified virus on growth of the virus-free parasite were studied. A cloned WB strain, sensitive to the viral infection, and a cloned E-9/M strain, resistant to the infection, were studied. With the WB strain, infection can occur at a ratio as low as 10 viral particles per organism. As the virus to parasite ratio increased, the rate of growth of the parasite decreased and the percentage of parasites not adhering to the culture tube wall also increased. These nonadhering cells, which differed from the nonadhering cells under normal growth conditions, were unable to divide. They contained an average number of 500,000 viral particles per cell which may be the threshold intracellular density of viral particles arresting the growth of G. lamblia. The results also suggest that the specific consequence of viral infection, even at extremely high multiplicity of infection, is not lysis of G. lamblia trophozoites but cessation of growth.