Tanzania Journal of Agricultural Sciences 1999, Vol. 2(2): pp193- 204Micro-catchment Rainwater Harv.esting. (RWH) has been defined as a method of collecting run-off from a Catchment Area (CA) over short distances not exceeding 100 m and supplying it to an adjacent Cultivated Basin (CB). It is a system that is designed to concentrate rainwater so as to utilize it more effe"ctively in areas- where. the seasonal. rainfall amounts are frequently lower than crop water requirements. The Catchment to Basin Area Ratio (CBAR) is an important parameter in the design of micro-catchment systems. It usually varies between 1:1 and 10:1. However, methodsfor deciding the optimumle·ve! of CBAR for differef!t farming systems are not available. The purpose of the experiments reported here was to evaluate the CBARfor maize production in semiarid areas of Tanzania. The experiments were run between 1992 and 1995. in semi-arid areas of Morogoro and Mwanga Districts of Tanzania, to assess the peiformance of maize grown in microcatchment systems with CBAR varying from 0:1 to 4:1. Maize var. TMVI was grown in Mwanga District while maize-var. Staha was used as a test crop in Morogoro District. Grain was harvested in five out of six experimental seasons in Mwanga (Masika 1993, 1994 and 1995 and Vuli 199411995 and 199511996). In Morogoro, grain harvest was obtained only in two seasons (Masika 1993 and 1994) out offour. The results showed that micro-catchment RWH farming is feasible during Vuli. The yield benefits due to RWH were found to be 120 - 152· % and significant at P = 0.05. The benefits during Masika were found to be very low at only 12 - 17 % and not significant at P = 0.05.