Epitaxial Si 〈111〉/CoSi2/Si structures have been grown by molecular beam epitaxy. The transistor effect, via ballistic electron transport in the CoSi2 200 Å-thick base, has been observed. The ballistic mean free path in CoSi2 obtained (λB ̃ 105 Å) from the current gain is consistent within 20% with the predicted value (λBth ̃ 80 Å).