This paper devises a fiscal policy by means of which the first-best optimum equilibrium is attained as a market equilibrium in the Uzawa-Lucas model when average human capital has an external effect on productivity. The optimal policy requires the use of a subsidy to investment in human capital which can be financed by a tax on labor income. Lump-sum taxation is not required to balance the government budget either in the steady state or in the transitional phase. Physical capital income should not be taxed. Alternatively, the optimal growth path can be attained by means of a subsidy to human capital.