We study the subgap conductance of an NSN transistor, consisting of a
superconductor grain, connected to two normal electrodes by tunnel jun
ctions. Transport is due to electrons passing in pairs through the gra
in (Andreev reflection). We calculate the corresponding tunneling rate
under Coulomb blockade conditions. For charging energies E(c) smaller
than DELTA, the linear conductance has sharp peaks periodic in the ga
te voltage with a period twice larger than for a normal grain. If the
applied bias voltage exceeds a certain threshold value, trapping of a
single quasiparticle suppresses these peaks.