The orientation of muscle fibers influences the physiological cross-sectional area, the relationship between fiber shortening and aponeurosis shear, and the total force produced by the muscle. Such architectural parameters are challenging to determine particularly in vivo in multicompartment structures such as the human soleus with a complex arrangement of muscle fibers. The objective of this study was to map the fiber architecture of the human soleus in vivo at rest in both neutral and plantarflexed ankle positions using an MRI-based method of diffusion tensor imaging (DTI). Six subjects were imaged at 3 Tesla with the foot at rest in the two ankle positions. Eigenvalues, fractional anisotropy (FA), and eigenvector orientations of fibers in the different soleus subcompartments were evaluated after denoising of the diffusion tensor. The fiber architecture from DTI was similar to earlier studies based on a 3D fiber model from cadavers. The three eigenvalues of the diffusion tensor increased by ∼14% on increasing the joint plantarflexion angle in all of the soleus subcompartments, whereas FA showed a trend to decrease in the posterior and marginal soleus and to increase in the anterior soleus. The angle change in the lead eigenvector between the two foot positions was significant: ∼41° for the posterior soleus and ∼48° for the anterior soleus. Fibers tracked from the subcompartments support these changes seen in the eigenvector orientations. DTI-derived, subject-specific, muscle morphological data could potentially be used to model a more complete description of muscle performance and changes from disease.