Culture forms of Trypanosoma ranaruam in basic SNB-9 medium (pH 7.5 to 8.5) displayed greater numbers of pear-shaped, reproductive forms than in standard SNB-9 medium. Media on the acid side of neutrality (pH 5.5 to 6.5) contained large numbers of slender, nonreproducing forms. Transfer of alkaline culture forms to standard medium resulted in normal growth; cultures from acid media required a prolonged lag phase before normal growth was resumed. Alkaline phosphatase is not evident in acid media nor is acid phosphatase in alkaline media. After transfer to normal, standard medium, both enzymes could be detected by cytochemical means. The cyclic development and reproduction of the culture forms of Trypanosoma ranarum are modified by temperature (Lehmann, 1962). In the present work, investigations were made upon the effect of pH on growth, development, and content of alkaline and acid phosphatases in the cultural forms of T. ranarum. MATERIALS AND METHODS Stock cultures of T. ranarum were grown in Diamond's (1954) SNB-9 medium. Both the overlay and solid phase of the SNB-9 medium were adjusted to the experimental pH levels by the addition of 0.1 N HC1 or NaOH; the following levels of pH were employed: 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, and 8.5. Immediately after the overlay was added, in 2.0-ml amounts, 0.2 ml of 20-day stock culture was inoculated and the number of flagellates counted under a hemocytometer; the final numbers of flagellates averaged 1.5 X 106/ml of overlay. The numbers of trypanosomes were determined daily as were the morphological types present; the latter check was made by observation of a Giemsa's stained smear. Presence of the enzymes acid and alkaline phosphatase was determined by the naphthol method of Burstone (1958). Cultures were examined daily for 4 days and then every other day until 12 days, the termination of the experiments. All experiments were run at room temperature. Triplicate tubes were made for each experiment and other tubes were made for the express purpose of determining pH; pH was checked with a BeckReceived for publication 15 March 1963. * This work was supported by a research grant (AI 05124) from the National Institutes of Health, U. S. Public Health Service. man pH meter at inoculation and after 1, 3, 7, and