Many characteristics of nanoscale materials are size-and shape-dependent, including their optical, electronic, and catalytic properties. [1±4] Spherical gold nanoparticles are easily produced by the chemical reduction of gold salts. [5,6] Anisotropic metal nanoparticles have been prepared using electrochemical [7] and photochemical [8] reduction methods in aqueous surfactant media, porous alumina templates, [9±10] polycarbonate membrane templates, [11] and carbon nanotube templates. [12,13] We recently reported the wet chemical syntheses of relatively monodisperse gold and silver nanorods, as well as silver nanowires. [14±16] Our method involves surfactantdirected growth of nanorods from spherical seeds. [17,18] Seeds are prepared by reduction of a metal salt with a strong reducing agent; the growth steps involve the addition of more metal salt to the seed solution, with a weak reducing agent, in the presence of a surfactant. The aspect ratio of the nanorods can be controlled by varying the ratio of seed to metal salt [14] as well as the time delay between steps of the syntheses. Gold nanorods with aspect ratios of 4.6 ± 1.2, 13 ± 2, and 18 ± 2.5 have been prepared reproducibly by our method. [14] One drawback to this method is that it requires extensive separation of nanorods from spherical and plate-like gold nanoparticle side-products (Fig. 1a), achieved by up to a half-dozen rounds of centrifugation and washings. The separation of gold nanorods from other shapes is time consuming, and the yields of gold nanorods are low,~4 %. [14] We here report an improved synthetic methodology that produces monodisperse gold nanorods of high aspect ratio in~90 % yield in comparison to the initial method that produced gold nanorods iñ 4 % yield. Transmission electron microscopy (TEM) images of gold nanoparticles produced by the previous method and by the new method, before purification, are shown in Figure-s 1a and 1b, respectively.In our previous method, [14] gold nanorods were synthesized at pH 2.8. The method involved a three-step seed-mediated growth process. In the first step, 4 nm gold nanoparticles are produced by the reduction of HAuCl 4 with sodium borohydride in the presence of sodium citrate. The 4 nm gold nanoparticles were used as seed for the next growth steps. The seed is added to a growth solution containing the surfactant cetyltrimethylammonium bromide (CTAB), HAuCl 4 , and ascorbic acid. The ascorbic acid is the weak reducing agent, incapable 414