Microtubules of polyaniline (PANI) doped with HCl, HBF4, and beta-naphthalene sulfonic acid (NSA) were synthesized by an improved in situ doping polymerization method. Ultraviolet-visible spectra, scanning electron microscopy, ESR, and X-ray diffraction characterized the molecular structure of the resulting PANI microtubules. These microtubules had a diameter of 1 similar to 10 mu m and a conductivity at room temperature of 0.2 similar to 3.5 S/cm, depending on the molecular structure and concentration of the dopant. The degree of crystallinity of the PANI microtubules was enhanced by increasing the molecular size of the dopant; that is, the order PANI-NSA > PANI-HBF4 > PANI-HCl was observed in the crystallinity of the microtubules.