Field experiments were conducted in 2011 and 2012 at the Teaching and Research Farm of the Federal University of Agriculture, Abeokuta (Latitude 7 20`N and Longitude 3 23`E) in the Forest/ Savanna Transition Agro- ecological zone of South Western Nigeria to determine the weed control efficacy and AM colonization of upland rice varieties at different population densities. The experiment was a 3x3 factorial arranged in a Randomized Complete Block Design (RCBD) and replicated three times. The three plant population used included 333, 333; 250, 000 and 166, 666 plants per hectare obtained by planting at 20cm x 15cm, 20cm x 20cm and 20cm x 30 cm respectively, while the three rice varieties used were ITA 150, NERICA 2 and OFADA. The results obtained from the study showed that plant population at 333, 333 plants per hectare significantly (p < 0.05) reduced weed population, number of tillers per hill and yield of rice varieties. Moreover, the local rice variety, OFADA was observed to be significantly (p < 0.05) taller than the other varieties, reduced weed biomass and consequently gave better yield advantage over the other tested varieties (ITA 150 and NERICA 2). Panicle length and days to 50% flowering were significantly (p<0.05) longer in OFADA and NERICA 2. Planting density did not affect AM root colonization but significant differences were observed among the rice varieties with respect to spore abundance. Keywords: rice, arbuscular mycorrhizal, plant population, varietie