The effects of temperature and concentration on the fluorescence lifetime of the 3P0 excited state in praseodymium-doped ZBLAN glass were studied. Above a critical dopant concentration, at which the lifetime exhibits no temperature dependence, the fluorescence lifetime increases with increasing temperature while below this concentration, the fluorescence lifetime decreases with increasing temperature. This critical concentration is determined to be 6900±220 ppm from measurements of the lifetime for varying temperatures and concentrations.