Motivated by some hints of M24 Moonshine in the spectrum of BPS D-brane boundstates (stable objects in the derived category) on a K3 surface we investigate some special compactifications of the heterotic string for which the space of BPS states is, in a natural way, a representation of various subgroups of the Conway group. While our results indicate that there is no meaningful M24 Moonshine for the degeneracies of BPS D-brane boundstates on a K3 surface, the existence of Conway subgroup symmetric compactifications of the heterotic string has some interesting implications for D-brane boundstates on some K3-fibered Calabi--Yau manifolds.