ABSTRACTOwing to favourable soil and climatic conditions, the naturally forested highlands of Tanzania have a comparatively high potential for agriculture. Major limiting factors are drought damage and erosion. The severity of these depend on the interaction between land use practices, soil/ topography and climate. As a result of rapid population increase land use practices are becoming more intensive, resulting in deteriorating soil structure. This in turn will lead to changed relations between climatic features on the one hand and crop and soil damage incidence on the other. The report is based on analyses of rainfall, interception and evaporation data in a forest and a small clearing in the Mazumbai Forest Reserve in the Usambara Mts. in NE Tanzania, obtained from measurements carried out 1971–1976, and of monthly rainfall data from the period 1945–1976 from the nearby Mazumbai Estate. The mean annual rainfall in Mazumbai is 1227 mm but the variability is great. Standard deviation from the mean (based on a record of 32 years) is 324 mm and the rainfall likely to be exceeded in 4 years out of 5 is 950 mm. The highest annual rainfall recorded in the period is 1864 mm and the lowest 721 mm. April and May have high (180 and 250 mm) mean monthly rainfall, all other months have an average between 50 and 100 mm, but the variation from year to year is very high and any month, except April and May, may be completely dry. Prolonged droughts, i.e. two or more consecutive months with less than 50 mm of rain are frequent. In the 32-year period two very long such droughts have occurred (5 and 6 months), when only 30–35 % of normal rainfall was received.The longest dry spell (i.e. four or more consecutive days with less than 0.25 mm rain) recorded in the clearing lasted for 35 days. Spells of shorter duration are frequent and may occur any time, also during the long rains (April—May). Mean number of rain-days (>0.25 mm rain) per year in the forest clearing was 125, the maximum monthly value occurring in May with 20 rain-days and the minimum in January with 7 rain-days. The mean rainfall per rain-day is 10.3 mm (max. in May with 15.4 mm and min. in August with 4.9 mm). The highest daily rainfall recorded was 102.4 mm. Rain-days with less than 5 mm rain account for 50 % of the total no. of rain-days but only 11 % of the total rainfall. Total throughfall in the forest, i.e. rainfall at 0.5 m under closed canopy, during 2 1/2 years measurements, amounted to 78 % of rain received in the open. Neglecting stemflow, this gives a rain interception of the forest canopy of 22 %. Evaporation measured at 1.5 m height above ground with Andersson evaporimeters was five times higher in the clearing than in the forest–677 mm and 127 mm respectively per year. The seasonal variation in evaporation was much larger in the clearing than in the forest. In the final discussion it is pointed out that the incidence of land damage, in the form of erosion or reduction of crop growth, caused by climatic events such as intensive rains and prolonged droughts, will increase rapidly in the Usambara Mts., as a result of land use practices causing deterioration of soil structure.