Numerous Investigators have reported that NH₄⁺ uptake reduces the yield and quality of flue-cured tobacco (Nicotiana tabacum L.). Metalaxyl, a fungicide commonly used in the production of flue-cured tobacco, has been patented as a nitrification inhibitor. The objectives of this study were to 1) determine the influence of metalaxyl at commonly applied rates on the + nitrification of NH₄⁺ from various sources; 2) study the Influence of soil pH on inhibition of nitrification by metalaxyl; 3) study the Influence of N source and metalaxyl on N accumulation in the plant; and 4) evaluate the influence of N source and metalaxyl on the yield, quality, and chemical composition of flue-cured tobacco leaf. Field, greenhouse, and laboratory experiments were conducted in the Southern Piedmont region of Virginia in 1984 and 1985 to carry out these objectives. Metalaxyl was found to reduce the population of NH₄⁺ oxidizers in soil and inhibit nitrification at applications of 0.56, 1.12, and 3.36 kg ha⁻¹. The inhibitory effects of metalaxyl were much weaker than nitrapyrin. Nitrification was inhibited by metalaxyl only in soils where nitrification was slowed by low pH and wet conditions. In soils of high nitrifying capacity, metalaxyl did not inhibit nitrification. Nitrogen uptake was enhanced by high NO₃⁻ concentrations in the soil, except where metalaxyl reduced NO₃⁻ leaching. Cured leaf concentrations of N were not affected by N source but were slightly reduced by metalaxyl. Nitrogen source and metalaxyl did not affect yield, total alkaloids, or reducing sugars. Quality Index was reduced by decreased soil NO₃⁻ from both N source and metalaxyl. / M.S.