The purpose of this study was to examine the economic implications of a rapid population growth on Nigeria's economic development. It was particularly interesting to study the relationship, because at the present Nigeria is making some economic progress while undergoing a demographic transition. Apparently, despite the acceptable growth of the national income, the growth of the per capita output has not been encouraging. This output growth must have been hampered by the rather rapid population growth in Nigeria. The neoclassical growth theory was basically employed to explain the growth of output in the economy in terms of both capital and labor inputs. For instance, the short-run impact of a possible fertility decline could lead to increased savings capability, possible through the curtailment of the consumption of the dependent population. The long-run impact, on the other hand, could be the opportunity to increase the rate of structural transformation needed to raise labor productivity and personal income in the economy. The model revealed that economic growth rates in Nigeria have been declining with rising affluence. It is more likely that such a slowing would arise from the population pressure and resource limitation rather than from the propensity to invest.