The dislocation structures in silicon single crystals deformed up to different points in the yield region, at 823 K and at the shear strain rate γ of 2 × 10−5 s−1, have been estimated by transmission electron microscopy. In addition to the total dislocation density ρt, the mobile dislocation density ρm can be measured with reasonable accuracy, thanks to the characteristic shape of dislocation loops. Local shear stresses are estimated from the radius of curvature at dislocation bends. Estimation of the effective stress τeff is difficult before the lower yield point because of micro-scale inhomogeneities. The internal consistency of the measured τeff and ρ has been checked using Orowan's relation, leading to the conclusion that, in the experimental conditions realized, τeff is very close to the applied stress and nearly all dislocations are mobile from the onset of plastic deformation up to the lower yield point.