The molecular dynamics after the photoexcitation of 1 ,3 ,3 -trimethylspiro-8-nitro(2H-1-benzopyran-2 ,2 -indoline) (8-nitroBIPS) were investigated by the transient grating (TG) method. When the excitation laser power is low, the TG signal consists of three components which reflect the thermal diffusivity in the solution and the translational diffusion of the colourless and coloured forms of 8-nitroBIPS. The diffusion constant of the colourless form is found to be approximately 1.5 times larger than that of the coloured form, and this result is explained in terms of the photoinduced electric dipole moment of the coloured form. With stronger excitation light, another new TG signal is observed in the longer time region in non-polar solvents. This signal is attributed to the intensity grating produced by probe light scattering due to the microcrystals of 8-nitroBIPS. From the temporal profile of this new TG signal (aggregate grating signal), the dynamics of the aggregate formation process can be investigated.