Globular proteins may be stabilized, either intrinsically, at the various levels of the structural hierarchy, or extrinsically, by ligand binding. In the case of the dormant all-β protein spherulin 3a (S3a) from the slime mold Physarum polycephalum , binding of calcium ions causes extreme kinetic and thermodynamic stabilization. S3a is the only known single-domain member of the two Greek key superfamily of βγ-crystallins sharing the extreme long-term stability of its homologs in vertebrate eye lens. Spectral analysis allows two Ca 2+ -binding sites with K D =9 μM and 200 μM to be distinguished. Unfolding in the absence and in the presence of Ca 2+ gives evidence for extreme kinetic stabilization of the protein: In the absence of Ca 2+ , the half-time of unfolding in 2.5 M guanidinium chloride (GdmCl) equals 8.3 minutes, whereas in the presence of Ca 2+ , even in 7.5 M GdmCl, it exceeds nine hours. To reach the equilibrium of unfolding in the absence and in the presence of Ca 2+ takes one day and eight weeks, respectively. The corresponding Gibbs free energies (based on the two-state model) are 77 and 135 kJ/mol. Saturation of S3a with Ca 2+ leads to an upward shift of the temperature-induced equilibrium transition by ca 20 deg. C. The in situ Ca 2+ concentration in the spherules is sufficient for the complete complexation of S3a in vivo .