Practically all fluorescent proteins from corals are oligomerized or aggregated in solutions. For zFP538 it was demonstrated that intensity of fluorescence and pH-profile change upon dilution that was explained by the dissociation of aggregates. Particle size distribution of zFP538 aggregates adsorbed on the surface of graphite from solutions in the concentration range 10-7 - 10-9 M was studied by AFM. Multi-population size distribution was observed for all three protein concentrations. Within one concentration, size distribution is characterized by three populations: small, medium, and large. For 10-7M concentration, the largest size is 10 times less than that in the 10-5M solution or solid film. For 10-8M concentration, the maximum population is of medium size, but smaller in aggregation number than the minimal population for 10-7M. Aggregation numbers for small, medium, and large populations for 10-9M concentration are the same as for 10-8M, but the population is shifted to smaller aggregation number. Thus, upon dilution, an average aggregation number is gradually shifted to a smaller value. Difference in small, medium, and large particles is observed within 10-7 - 10-8M concentration range.