Gold nanorods were prepared via a seed-mediated sequential growth process involving the use of citrate-stabilised seed crystals and their subsequent growth in a series of reaction solutions containing [AuCl₄]⁻, ascorbic acid and the cationic surfactant cetyltrimethylammonuim bromide (CTAB). Electron diffraction analysis and HRTEM images of mature nanorods showed superpositions of two specific pairs of crystallographic zones, either and or and , which were consistent with a cyclic penta-twinned crystal with five {111} twin boundaries arranged radially to the [110] direction of elongation. The nanorods have an idealised 3-D prismatic morphology with ten {111} end faces and five {100} or {110} side faces, or both. TEM studies of crystals at various stages of growth indicated that the seed crystals are initially transformed by growth and aggregation into decahedral penta-twinned crystals, 4% of which become elongated when a fresh reaction solution is added, whilst the remaining twins grow isometrically. Reiteration of this procedure increases the length of the existing nanorods, induces further transformation of isometric particles to produce a second (and third) population of shorter, wider nanorods, and increases the size of the isometric crystals. The data indicate that symmetry breaking in fcc metallic structures to produce anisotropic nanoparticles is based on an intrinsic structural mechanism (twinning) that is subsequently modulated extrinsically during growth in solution by specific adsorption of Au^I–surfactant complexes on the side faces/edges of the isometric penta-twinned crystals and which is responsible for the preferential growth along the common [110] axis. We propose that the coupling of multiple twinning and habit modification is a general mechanism that applies to other experimental procedures (electrochemical, inverse micellar media) currently used to prepare metallic nanoparticles with a high aspect ratio.