Harmful microalgal blooms or red tides are often associated with high levels of pH. Similarly, species and strains of microalgae cultivated in the laboratory with enriched media experience recurrent events of high pH between dilutions with fresh medium. To study the potential for laboratory selection by high pH, we compared, under identical experimental conditions the upper pH tolerance limits for growth in addition to growth and production rates of 23 strains of the common bloom-forming dinoflagellate Heterocapsa triquetra. The strains had been cultivated in official culture centres from ca. 1 to 51 years (corresponding to 200–10,000 generations). Strains cultivated for less than 10 years had significantly lower mean and median upper pH tolerance limits for growth, and higher growth and production rates compared to strains cultivated for more than 20 years. The range and variation of upper pH tolerance limits were higher in the younger (20 years). These results suggest selection of strains best adapted to tolerate or postpone/avoid events of high pH in the laboratory. Our data have implications for experimental studies of pH response and reaction norms in general of microalgae and the inclusion of species-specific data into ecosystem models.