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The quantum size, impurity position and electric ¢eld e¡ects on the energy of a shallow donor placed anywhere in a GaAs spherical quantum crystallite embedded in Ga 1Àx Al x As matrix are studied theoretically in the framework of the e¡ective mass approximation. The impurity is supposed on the z-axis. The conduction band o¡set between the crystallite and the matrix is assumed to be ¢nite. The ground state energy and the spatial extension of the unperturbed hydrogenic donor are determined by Ritz's variational method. It appears that for a ¢xed size of the crystallite, the energy and the spatial extension increase monotonically when the impurity is displaced from the centre to the surface of the crystallite. The in£uence of a uniform electric ¢eld F is analysed. It is found that for a donor placed at the centre of the crystallite, the energy level is shifted to low energies. However for an o¡-centre donor the energy level shift depends on the angle between F and z.