Dronpa is a photoswitchable protein from the family of green fluorescent proteins (GFPs). Photoswitching involves forward and backward proton transfer between a bright deprotonated form (B) and a dark protonated form (A(2)). We have used femtosecond transient absorption spectroscopy to determine the kinetics of the first step of the photoconversion from the dark to the bright form, which we have measured as 4 ps. The 2-fold isotope effect on the kinetics of this process shows that excited-state proton transfer (ESPT) is involved in this step. It was also demonstrated that the acid-induced protonated form A(1) and the photoconverted protonated form A(2) are two distinct spectroscopic species.