Ultraviolet flow dichroism of brain microtubule was measured. Large positive dichroisms were observed at 255 nm, 285 nm and 292 nm. The dichroism at about 255 nm is due to bound GDP or GTP. The dichroism at 285 nm is due to tyrosine and tryptophan residues, and that at 292 nm is due to the tryptophan residues of the protein. These results show that the electronic transition moments of nucleotides and aromatic groups are both nearly parallel to the polymer axis.