Ignoring the stability effect of a conducting shell or conductors, it is found that an oblateness parameter ∊ =−0. 3 and an aspect ratio closest to one give optimal stability properties for a spheromak. In the pressureless case, only the n = 1 tilting and, possibly, the n = 2 kink modes are unstable. If these two modes are actively stabilized, appreciable beta values of a few per cent can be obtained, the limiting modes being Mercier modes with very small growth rates.