One of the unique features of the Development of Sustainable Aquaculture Project of the WorldFish Center, Bangladesh, was the provision of 3 y of continuous training and extension in support of the fish farmers involved. It was expected that the long-term support would make a significant contribution to boosting production and incomes as well as sustainability. The article demonstrates the impact of the training and extension support to small-scale carp polyculture farmers in terms of productivity, profitability, and efficiency. The study used data collected from 225 project farmers and 123 control farmers during 2002/2003 to 2005/2006. The results show that the project farmers achieved an average growth rate in productivity terms (kg/ha) of about 23% per annum compared with only 3.8% among control farmers. Estimated annual return on investment per demonstration farm exhibits 52% higher return over the 3-y intervention period. During 2002/ 2003 to 2005/2006, the average technical efficiency score of project farmers increased from 0.55 to 0.86 compared with an average increase from 0.67 to 0.78 among control farmers over the same time period. The results showed that the efficiency gains and resulting increased productivity came from the development of human capital through long-term efficient and effective training and extension support.