Abstract. The magnetic moment of fast ions confined in axisymmetric low-aspect-ratio spherical torus (ST) plasmas appears to be non-conserved due to the relatively strong inhomogeneity of the magnetic field B. We evaluate the magnitudes of non-adiabatic as well as adiabatic variations of the magnetic moment (MM) of fast ions in STs as effected by the flux surface (FS) shape and/or by small-scale poloidal modulations of B along the field line. Our consideration is based on a model magnetic field with non-circular but poloidally smooth FSs (V.Yavorskij et al., Pl. Phys. Contr. Fus. 43 (2001) 249), which has been extended to account for small-scale poloidal modulations of B that are typical for high beta ST equilibria. The strong sensitivity of non-adiabatic variations of the MM to weak small-scale poloidal perturbations of FSs and B is demonstrated. The investigation presented helps to understand the nature of the enhanced non-adiabaticity observed in the numerical EFIT field of NSTX, and it enables the evaluation of the accuracy of analytical estimates of nonadiabatic changes of MM. The effect considered is expected to substantially impact fast ion transport in STs and stellarators.