The effect of reduced solar radiation on associative N2-fixation and plant parameters was studied in three field experiments (1978–80). ‘Gahi-3’ pearl millet (Pennisetum americanum (L.) K. Monch.) field plots were shaded with saran shade cloth that reduced solar radiation by 50% and 75%. Acetylene reduction activity (ARA) was reduced by shading in one of the three experiments. The two non-responding experiments were conducted on a wall-drained, low-activity site (ARA means ranging 17–68 n moles ethylene core−1 h−1), the responding experiment was conducted on a poorly drained, high-ARA site.