Thermallyirreversiblephotochromic properties of indolylfulgenates, i.e. 2-alkylidene-3-[1-(1,2-dimethyl-3-indolyl)alkylidene]succinate esters were examined. A large indolylalkylidene group reduced the relative ratio of the colored species at the photostationary state of UV irradiation. Introduction of an adamantylidene group increased the bleaching quantum yield. The absorption maximum wavelengths