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The effect of different structure-directing agents (SDAs) on the nucleation of MTW (the structure code for ZSM-12 recommended by the International Zeolite Association Structure Commission) is shown by means of transmission electron microscopy (TEM). A loose-fit SDA interacting only weakly with the framework seems to have pore-filling character only and results in large domains with rather perfect ZSM-12 framework structure. Conversely, a more tight-fit SDA interacts markedly with the framework during the crystallization and causes simultaneous growth of small domains as well as frequent twinning. By comparison of simulated and experimental electron diffraction patterns as well as high-resolution transmission electron microscopy (HRTEM) images, the proposed crystal structure of ZSM-12 is substantiated. The presence of an organic SDA confined in the one-dimensional channels is established in HRTEM images obtained from calcined as well as as-synthesized specimens. This result is corroborated not only by corresponding image contrast simulations, but also by a significant reduction in intensity of low-angle reflections in quantitative electron diffraction patterns obtained from the as-synthesized material.