Experiments aimed at studying the role of microorganisms in the formation of kaolinite from aluminosilicate solutions (Si:Al = 1:1) are reported. The experiments were carried out at room temperature in presence of living microorganisms, Leonardite humic acid, bacterial debris, bacterial exopolysaccharides (EPS), and some organic ligands. The bacterial debris, EPS, Leonardite and organic ligands were chosen to stabilize Al in octahedral coordination for allowing the crystallization of kaolinite. A microbial population inherently contaminating a Leonardite standard, a bacterial population extracted from peat soil, and a strain of the fungus Paecilomyces inflatus were used. Microscopic observations on the precipitate collected from the reacting media were performed after 3 and 70 months of incubation. All the abiotic experiments failed to show any sign of kaolinite formation even in the long term. As far the biotic experiments are concerned, it was observed that i) the Leonardite standard, together with its inherent microbial population, was unable to play any role in precipitating kaolinite; ii) the bacterial population extracted from a peat soil favoured the formation of a few, small (<500 nm) and isolated pseudo-hexagonal kaolinite crystals; iii) the fungus P. inflatus contributed to the crystallization of large (even more than 2 μm) euhedral kaolinite crystals after 3 months of incubation. These findings confirm the hypothesis that bacteria and fungi are capable to produce or greatly accelerate kaolinite crystallization. The mechanism underpinning such a process is far from being fully understood.