The axenic cultures of two marine species of diatoms S. costatiun and T. fluviatilis were grown in the laboratory to study the pH tolerance in individual batch cultures and species competition in mixed cultures. The experiments were performed at varying pH levels in the range 7.0 to 10.0. Of the two species T. fluviatilis was hardly affected by varying pH up to its maximum tolerable level of 9.5, whereas the growth of S. costatum was adversely affected by increasing pH and ceased when the pH exceeded 9.0. However the growth rate was more or less same for both the species in the optimum pH. The optimum range of pH for S. costatum and T. fluviatilis is 7.5 to 8.2 and 8.0 to 8,5 respectively.

In the mixed culture, in all the pH ranges T. fluviatilis dominated over S. costatum. The growth rate of S. costatum was reduced very much and ceased when the pH exceeded 8.5 where as in T. fluviatilis the growth rate was maintained as in individual batch culture. This dominance results because of T. fluviatilis unique ability to tolerate high alkaline conditions and allelopathic interaction with the other species.