Monreal, M. A., Grant, C. A., Irvine, R. B., Mohr, R. M., McLaren, D. L. and Khakbazan, M. 2011. Crop management effect on arbuscular mycorrhizae and root growth of flax. Can. J. Plant Sci. 91: 315-324. The effects of two tillage systems, two preceding crops and the use of phosphorus (P) fertilizer on arbuscular mycorrhizal fungi (AMF) colonization, root biomass and root size of flax (Linum usitatissimum L.) were evaluated at two sites in Manitoba, Canada. The Brandon Research Centre Farm site had been historically managed under conventional tillage (CT), and the Manitoba Zero Tillage Research Association Farm site under reduced tillage (RT). The preceding crops were a mycorrhizal crop, wheat (Triticum aestivum L.), and a non-mycorrhizal crop, canola (Brassica napus L.), grown under RT or CT using rates of 0, 11 and 22 kg P ha^-1 applied as monoammonium phosphate (MAP). Phosphorus was applied to the following flax crop as MAP at rates of 0 or 11 kg P ha^-1. Flax root biomass was larger in all site-years when wheat rather than canola was the preceding crop. Also, AMF colonization and root size of flax increased in 3 and 2 site-years, respectively, when grown after wheat. Flax's root biomass decreased under CT and AMF colonization decreased when P was added either to the previous crop or to flax only in one site year. Weather conditions may have enhanced the effects of preceding crop, P fertilizer and tillage.