A molecular dynamics (MD) simulation study of a n-decyltrimethylammonium chloride micelle in water is presented. The model system contains 30 amphiphile molecules, 30 chloride ions, and 2166 water molecules. We use the same model potentials as in a recent MD study of a n-hexadecyltrimethylammonium chloride monolayer in water (Bocker, J.; Schlenkrich, M.; Bopp, P.; Brickmann, J. J. Phys. Chem. 1992, 96, 9915). The main objective of this work is to analyze the shape and the structure of the micelle. A slightly prolate ellipsoidal shape of the micelle emerges from a simulation of 275 ps. It is found that the interior is completely "dry". This result is in contrast to monolayers-aqueous solution interfaces (Bocker, J.; Schlenkrich, M.; Bopp, P.; Brickmann, J. J. Phys. Chem. 1992, 96, 9915). A molecularly sharp interfacial region appears, in agreement with experimental studies. The charges on the surface of the micelle are only partially neutralized by chloride counterions in the first solvation shells of the head groups. The area per head group on the micelle surface and the orientations of the alkyl chains as obtained from the simulation are in accord with experimental results.