A modified homogeneous precipitation technique involving enzymatic decomposition of urea was proposed and was applied to the synthesis of alumina precursors. Spherical aluminum hydroxide aggregates were prepared at 298 K from aqueous solutions containing aluminum sulfate, urea and urease. In contrast to the conventional technique involving thermally induced decomposition of urea, the resultant aggregates were amorphous with no trace of pseudoboehmite. The apparent rate of nucleation of the aggregates was found to correlate with the concentration of urease. As a result, the mean size of the aggregates could be controlled in a range between 0.3 and 1.9 mu m by adjusting the urease concentration. The precursors remained amorphous after calcination at 1073 K for 2 h, then transformed to gamma-alumina after being calcined at 1273 K. The transformation to alpha-alumina was completed after calcination at 1473 K.