Flagellated envelopes of Halobacterium salinarium cells were prepared by lysis with taurodeoxycholate. After solubilization of the envelopes with Triton X-100 at high ionic strength, flagella and round patches from which numerous flagella emerged were isolated by gel filtration chromatography. We conclude that the flagellar bundle of H. salinarium is inserted into a differentiated polar cap structure.