Sphaerophorus necrophorus was grown in Medium 156 at 40 pH values ranging from 4.93 to 9.46. Growth occurred between pH 5.30 and 9.28. The optimum pH for maximum growth was 6.84 while the optimum pH for acid production was 7.70. At pH 7.70 acid production was more than double that at pH 6.84 during a 12 hour incubation period. Thus, maximum growth occurs at one pH and maximum acid producation at another. Since the two optima are different, acid production should be evaluated only when: 1. an optimum medium is employed, 2. the optimum pH for acid production is employed. 3. the incubation period necessary for maximum acid production has elapsed before the test is conducted.