The detection of bifurcation in granular soils is a major concern in geomechanics. Recent advances in geomechanics have highlighted the key role played by the mesoscopic structures composed of grain loops in the stability of granular media. Based on an elementary grain loop pattern, this paper investigates the influence of the microstructure on the bifurcation domain topology at the mesoscopic scale. This study shows that, at the mesoscopic scale, the bifurcation domain cannot be consistently defined in terms of the stress state only. Expressed in a stress-microstructure space, the bifurcation domain becomes invariant with the contact regime, the initial microstructure, and the loading path. It is also shown that the microstructural anisotropy is as determinant as the stress state with respect to the bifurcation domain topology. Finally, the mechanical and volumetric behaviors of the granular assembly are shown to be correlated through a proper microstructural variable. (C) 2018 American Society of Civil Engineers.