We present a quantitative investigation by atomic-force microscopy of the growth of an organic thin film made of sexithienyl evaporated on mica in high vacuum. Different forms of aggregation in sexithienyl thin films are induced upon a systematic increase in the temperature of the substrate during the evaporation. For temperatures up to 150 \ifmmode^\circ\else\textdegree\fi{}C, the films consist of tightly packed grains whose size increases with temperature according to an Arrhenius behavior with \ensuremath{\sim}0.36 eV activation energy. The \ensuremath{\chi}-square grain-size distribution suggests that growth rate is controlled by diffusion. This mechanism is supported by a simple model calculation of the van der Waals interaction between a single T6 molecule and a (100) T6 surface, that yields energy barriers to translation and rotation of the molecule equal to 0.3 and 0.5 eV, respectively. At 200 \ifmmode^\circ\else\textdegree\fi{}C the film undergoes a morphological change to a lamellar structure and extended microcrystalline structures appear at higher temperatures. The presence of preferential directions suggests that orientational ordering is induced by the mica substrate. The possibility of obtaining ordered aggregates by a suitable choice of substrate and temperature during evaporation has relevance towards the realization of molecular devices with anisotropic properties.