CalGreen 1 is a cell-impermeant fluorescent calcium indicator that is characterized by high quantum yield and low phototoxicity. Its peak excitation and emission wavelengths (506 and 531 nm, respectively) are comparable to standard fluorescein dyes, making CalGreen 1 appropriate for fluorescent microscopy. CalGreen 1 is ~5-fold brighter than fluo-3 at saturating calcium levels.