White matter parcellation from Diffusion Magnetic Resonance Images reveals detailed 3D white matter structure and connectivity of the brain in vivo and thus is a viable tool for investigating neural differences. Measurements derived from these neural tracts or fascicles from Diffusion Tensor Images (DTI) exhibit potential as biomarkers for neurologic and neuropsychiatric disorders. - - - - - - - - - - - - - - - - - - - - We develop and implement a parcellation method that aims for lower sensitivity to initialization and local orientation error by directly segmenting full white matter tracts, rather than reconstructing individual curves, from diffusion tensor fields. A fast, robust volumetric and intrinsically normalized solution is achieved by efficient sampling and noise-filtering using a generic parametrized tract model. Interactive tract modification schemes are also developed to resolve ambiguity in branching tracts or to perform regional investigation for long fascicles.