The porphyrin trans-bis( N-methylpyridinium-4-yl)diphenylporphyine ( t-H 2P agg) is known to form in solution large assemblies and supramolecular structures in the presence of polyanions, such as nucleic acids, as a template support. We have studied the aggregation of the unbound porphyrin in 0.1 M NaCl aqueous solutions. The aggregation process was monitored by a time resolved small angle light scattering apparatus. The intensity profile of the scattered light as a function of the exchanged wavevector q, conforms to a scattered law for fractal aggregates in the range 0.2–31.4 μm −1. An analysis of the intensity data reveals a value of the fractal dimension d f equal to 2.5. Additional dynamic light scattering measurements confirm the presence of almost monodispersed aggregates in the mesoscopic range.