Symbiotic N₂ fixation reduces use of N fertilizers, thereby decreasing agricultural costs and environmental pollution. The manner in which acidic soils affect N₂-fixing symbioses is complex. Toxicities and deficiencies associated with acidic soils can affect symbiotic N fixation by such means as reducing multiplication of rhizobia in soil, preventing nodule formation, limiting nodule function, and affecting the host plant. It is important to determine the most significant acidic soil factor and whether it is affecting the host plant or the symbiosis. The symbiosis generally appears to be more susceptible to Al toxicity, low pH, and Mo deficiency than the host plant; however, the sensitivity of the plant may also be important, particularly regarding P deficiency and Mn toxicity. Increasing Ca can often offset inhibitory effects of low pH and Al. Sensitivities of symbioses to acidic soil factors vary with each plant/rhizobia species combination, and there is considerable variability in tolerance among rhizobia strains and plant cultivars of individual species. Monitoring multiplication of rhizobia in a defined medium has generally proven effective as a way of screening rhizobia for tolerance to acidic soil factors. Additional research is needed to develop a simple accurate method for measuring solubilized, toxic Al and to examine the effects of Mo deficiency and Mn toxicity on symbiotic N₂  fixation.