The Torus II high-beta tokamak has studied plasmas with ⟨β⟩ in the range 9–13%. Both stable and unstable equilibria were observed; in the case of unstable equilibria, the instability had the characteristics of high-toroidal-mode-number ballooning. The experimental equilibria were modelled computationally and tested for stability. Predicted growth rates and critical mode numbers are in reasonable agreement with observation.