The primary phase of transportation at the smallholder level, from village to local market, is a particularly important aspect of increasing agricultural production in developing countries. The realistic prediction of vehicle operating costs on the (largely) unsurfaced roads in this sector is a useful input to development planning and a computer program has been developed to produce such predictions from first principles. When compared with survey results obtained by the Transport and Road Research Laboratory in Kenya, it is found that correlation is satisfactorily close. The program can also be used to predict the effects, on the operating costs of various vehicles, of changing road characteristics (gradient, curvature, roughness, rolling resistance and traction). It is found that rolling resistance and road roughness are the factors most likely to influence operating costs, due to their effects on vehicle speeds, fuel consumption and service/repair costs. Small, cheap machines are not necessarily superior to larger vehicles in terms of costs per tonne kilometre and fuel, particularly where the available load is sufficient to allow the larger vehicle to be utilized reasonably fully.