In eukaryotes, changes in cytosolic Ca2+ concentrations ([Ca2+]cyt) are associated with a number of environmental and developmental stimuli. However, measuring [Ca2+]cyt changes in single plant or algal cells is often problematic. Although a wide range of Ca2+-sensitive fluorescent dyes is available, they are often difficult to introduce into plant cells. Micro-injection is the most robust method for dye loading, but is time-consuming, technically demanding, and unsuitable in many cell types. To overcome these problems, we have adapted biolistic techniques to load Ca2+-sensitive dyes into guard cells of the flowering plant, Commelina communis, cells of the green alga Chlamydomonas reinhardtii, and zygotes of the brown alga, Fucus serratus. Using this approach, we have been able to monitor [Ca2+]cyt changes in response to various stimuli, including a novel [Ca2+]cyt response in C. reinhardtii. The method allows the use of free acid and dextran-conjugated dyes. Biolistic loading of differentiated plant cells is easier, quicker, and more widely applicable than micro-injection, and should broaden the study of plant signal transduction.