Turmel, M. S., Entz, M. H., Tenuta, H., May, W. E. and LaFond, G. P. 2011. The influence of a long-term black medic (Medicago lupulinacv. George) cover crop on arbuscular mycorrhizal fungal colonization and nutrient uptake in flax (Linum usitatissimum) under zero-tillage management. Can. J. Plant Sci. 91: 1071-1076. Leguminous cover crops are becoming a popular way to increase the sustainability of agricultural systems. Previously, cover crops have been found to increase colonization by arbuscular mycorrhizal fungi (AMF) and phosphorus and micronutrient uptake. Long-term field studies were conducted to test the hypothesis that self-regenerating black medic (Medicago lupulina cv. George) cover crops increase AMF colonization and early nutrient uptake in flax (Linum usitatissimum). Field experiments were established in 2000 (Manitoba) and 2002 (Saskatchewan) using a flax-wheat (Triticum aestivum)-oat (Avena sativa) rotation. In a second experiment, intact soil cores were harvested from the plots in spring and tested for soil disturbance and cover crop effects under controlled environment conditions (CEC). Both seedling flax crops sampled from the field in 2005 and 2006 and flax growth in CEC showed high levels of AMF root colonization, but no significant influence of the cover crop on AMF colonization by arbuscules or hyphal structures was detected. The AMF enhancing practices used in the experiments (i.e., zero-tillage and inclusion of mycorrhizal crops) may have contributed to the lack of cover crop effect on AMF colonization. The cover crop had no effect on macro- or micronutrient uptake by flax except during drought conditions (Winnipeg 2006), where flax biomass was reduced by 38% and the total uptake of N, P, Zn and Cu was decreased by 34, 30, 31 and 35%, respectively, in the medic treatment.