The purpose of this investigation was to determine the effects of soil pH and rates of nitrapyrin and metalaxyl on nitrification and growth and mineral composition of flue-cured tobacco under greenhouse conditions. Soil was amended with a complete NPK fertilizer for optimum tobacco growth where the nitrogen (N) was added as 300 mg N kg^-1 from (NH₂SO₄ and varying rates of Ca(OH)₂ to achieve the acidity levels of pH 5, 6, 7, 8, and 9. Nitrapyrin was added at 0, 0.25, 1 .0, and 2.0 mg a.i. kg^-1 and metalaxyl at 0, 2.5, and 0.5 mg a.i. kg^-1 to all pH treatments. There were three replications each of treatments with and without plants. At regular intervals during a 60-day period, soil without plants was assayed for changes in pH and ammonium and nitrate nitrogen levels. In companion containers, tobacco (cv. Speight G-28) growth was monitored for 60 days, and the mineral composition of the plants was determined at the end of the experiment. Increasing levels of nitrapyrin retarded the rate of nitrification. Correspondingly, the levels of ammonium were elevated and acidification of the soil was not as great as compared to pots with little or no nitrapyrin. Metalaxyl had no effect on nitrification. The rate of nitrification was reduced by soil acidity and ammonium remained higher for longer periods of time in acid soil treatments. There was no interaction of acidity with the nitrification inhibitor. The dry weight of tobacco plants was greatest at pH 6, 7, and 8, somewhat depressed at the most acid condition, and strongly inhibited at pH 9 compared to the pH levels of 6, 7, and 8. Although total N uptake of plants growing at pH 5 was similar to that of plants grown at a more favorable pH, the plants at pH 5 showed striking foliar abnormalities. We interpreted this to mean that tobacco was undergoing ammonium toxicity. At pH 9, plants were considerably smaller than at the other pH levels and low total N values were suggestive of an inadequate N level. Soil analysis of ammonium revealed significantly lowered levels, probably due to ammonia volatilization as a consequence of the high pH.