The bloom forming marine cyanobacterium Nodularia spumigena Mertens produces the toxic compound nodularin. The production and excretion of nodularin during growth were measured in a monoculture of N. spumigena and in a mixed culture with the cryptophyte Rhodomonas salina. Most of the toxin produced was found intracellularly, and extracellular nodularin was not detected until the stationary growth phase. The presence of R. salina did not significantly affect the production and excretion of nodularin by N. spumigena. Addition of pure nodularin did not affect the growth of a monoculture of R. salina. Similarly, no allelopathic effects were found when N. spumigena was tested in mixed culture experiments with 6 different phytoplankton species. Instead of being inhibited by an allelopathic compound, the growth of the test algae was controlled by the pH level in the culture media and by the pH tolerance of each species involved. Because N. spumigena can elevate pH up to very high levels (10.6), it out-competed 5 of the 6 phytoplankton species. This suggests that the high pH tolerance of N. spumigena in natural environments might be important for N. spumigena in the competition with other phytoplankton species.