Two media have been devised: an enriched seawater medium for culture of dinoflagellates and a defined medium for rapid growth of the dinoflagellate Cachonina niei. A wide range in salinity (10.23–42.38 g/liter NaCl) is tolerated by C. niei. Below 0.6 g/liter MgSO4, 0.19 g/liter KCl, and 0.22 g/liter CaCl2, the generation time greatly increases. Increase in MgSO4 to 7.22 g/liter, KCl to 1.12 g/liter or CaCl2 to 2.22 g/liter has little effect on generation time. The temperature optimum is 19–23 C. Saturating light intensity for growth is 1000 ft-c and for photosynthesis (determined manometrically) is slightly less than 2000 ft-c. Cachonina niei requires B12 and thiamin. Neither silicate nor its competitive inhibitor germanate affects generation time or cell yield indicating silicon is not required. Of a variety of buffers tested, Tris is the best. Optimal growth occurs at pHs of 7.5–8.3. Glycerol is inhibitory and does not support dark growth.