The localization and storage of alkaloids were investigated in a low producing cell suspension culture of Catharanthus roseus(L.) G. Don. (Apocynaceae). Fluorescence microscopy and electron microscopy indicate alkaloid accumulation to occur inside the vacuoles of particular cells. These alkaloid storage cells exhibit a vacuolar pH of 3, while “normal” cells of a suspension culture have a vacuolar pH of about 5. Alkaloids are taken up in their unprotonated forms, trapped by protonation inside the vacuole and are accumulated there. The differentiation of alkaloid storage cells depends both on the cell line and the growth conditions and seems to be a prerequisite for the accumulation of alkaloids in cell suspension cultures of Catharanthus roseus.