The relationship between the stability properties of ideal low-n internal modes and the threedimensional (3-D) ideal Mercier criterion for ℓ = 2 torsatron configurations is discussed. A broad range of aspect ratios and pressure profiles are considered. It is found that the low-n modes with resonant surfaces lying in a Mercier unstable region are unstable in most of the cases studied and that the critical beta given by the Mercier criterion is always lower than the critical beta for the lowest-n unstable mode. This is verified even in the cases where global n = 1 modes are unstable. Therefore, the 3-D Mercier criterion is a useful guide in mapping the ideal stability beta limits for these torsatron configurations.