Agriculture is increasingly practised on the very steep slopes of mountainous Vietnam with serious problems of soil erosion and degradation. In five Black Thai villages of Yen Chan and Mai Son district, northwest Vietnam, soil parameters and crop yields of 19 maize (Zea mays L.) and 25 cassava (Manihot esculenta Crantz) fields with 33-91% inclination at upper and lower mid slope positions were studied. Farmers' preferences for different fanning systems components was assessed by interviews in Black Thai, Hmong, Xinh Mun and Khmu villages and soil conservation strategies evaluated. The 10-22% lower organic matter, nitrogen and phosphorus content at the lower mid slope was associated with decreased maize yields by 27% and cassava yields by 31%, compared to upper mid slope positions. This accelerated soil degradation at lower slope positions may primarily be attributed to an enhanced mineralisation and crop export rather than to soil erosion due to more frequent historic cropping activities towards lower slope positions. The general implication to toposequence studies for assessing erosion. effects is that they are likely to be confounded with historic farmers' preferences to cultivate easier accessible, lower slope positions. Irrespective of the causes of degradation, soil fertility was not a priority for farmers whose cropping management is currently focusing on the introduction of improved maize varieties. Consequently, only soil conservation and fertilisation strategies that include long- and short-term interests of farmers should be promoted in the future: vegetative barrier and cover crops with food and fodder species, fruit and timber trees and minimum tillage. (C) 2002 Elsevier Science B.V. All rights reserved.