Mwea is the most highly sophisticated and successful vice scheme undev organised irrigation in Africa. Its production of 3 tons of paddy per acre ranks with that of the highest rice producers in the world. The scheme 's mayor aim is to settle landless and unemployed peasants on land which has been provided with drainage and irrigation3 and which is capable of intensive agricultural production. It is a combination of state agriculture and. tenancy under a licence agreement. Mwea started from scratch in 1955. It is a constantly changing phenomenon in its sociological, economic3 environmental and ecological setting. The objectives of this study were directed to find out why the Mwea was a successful irrigation scheme; to discover what makes a successful paddy farmer; to ascertain the constraints on the future development of Mwea; and to determine whether the Mwea experience and success can be repeated elsewhere. Tenants were classified according to yield per holding} into high3 average3 and low-yielding groups. Correlation analysis was used to identify the major social and economic factors, which have resulted in the success or failure of the tenants. Soil alkalinity is a major problem, and is spreading. In the past several years, substantial acreage has had to be abandoned. Mwea has been over-extended and is presently pushed beyond its capability. The irrigation system was designed to serve approximately 12,000 acres. The addition of a new section of 2,000 acres involves a calculated risk, as the hydrological system is new in a state of disequilibrium. It is advisable to solve existing problems rather than to rush headlong into expansion. Problems often breed other problems. The lack of hired and/or casual labour at critical periods of harvesting and transplanting is now an acute problem. The statistical analyses and field enquiry substantiate that tenants in the low-yielding group are unable to mobilise their household labour. It is not the size of the household that matters, but the ability of the tenant to organise and mobilise it. Unemployment and underemployment, particularly of the youth, are mayor problems facing Mwea. Population build up at Mwea is increasing at an alarming rate. Mwea is economically sound, but sociologically a failure. The substantial profits from the Settlemer.1• have been used by the NIB to prop up failing irrigation schemes in Kenya. But benign neglect over the years makes it imperative that some of the profits are used to counteract the deteriorating social conditions at Mmea. It is through identifying the economic, social and geographic aspects which resulted in success or failure of the tenants at Mwea and by recognising the vuVnerability of Mwea that this study makes its contribution.