A process for preparing sterile crystals of aripiprazole monohydrate of desired small particle size and narrow particle size distribution, which comprises: (a) providing a jet stream of a solution of aripiprazole in an organic solvent, wherein the solution of aripiprazole in the organic solvent contains from 0.01 to 0.1 kg/L aripiprazole, and wherein the jet stream of the solution of aripiprazole in the organic solvent is heated at a temperature ranging from 70°C to 85°C; (b) providing a jet stream of the anti-solvent water, wherein the jet stream of the anti-solvent is heated at a temperature ranging from 2°C to 40°C; (c) causing the jet stream of the solution of aripiprazole in the organic solvent and the jet stream of water to strike each other and to impinge on one another at a flow rate of each which is the same or different but is within the range from 0.2 to 0.3 kg/min where jet nozzles of 0.02 inch internal diameter are employed to create high turbulence at their point of impact to achieve high intensity micromixing of each stream prior to nucleation, to produce a slurry of crystals of aripiprazole monohydrate; and (d) recovering crystals of aripiprazole monohydrate of desired small particle size and narrow particle size distribution, wherein 10% of the crystals of aripiprazole monohydrate produced have a particle size of less than 10 microns, 50 % of the crystals of aripiprazole monohydrate produced have a particle size of less than 35 microns and 95 % of the crystals of aripiprazole monohydrate produced have a particle size of less than 100 microns, and wherein said crystals of aripiprazole monohydrate have a mean particle size within the range from 25 microns to 100 microns. (a) providing a jet stream of a solution of aripiprazole in an organic solvent, wherein the solution of aripiprazole in the organic solvent contains from 0.01 to 0.1 kg/L aripiprazole, and wherein the jet stream of the solution of aripiprazole in the organic solvent is heated at a temperature ranging from 70°C to 85°C; (b) providing a jet stream of the anti-solvent water, wherein the jet stream of the anti-solvent is heated at a temperature ranging from 2°C to 40°C; (c) causing the jet stream of the solution of aripiprazole in the organic solvent and the jet stream of water to strike each other and to impinge on one another at a flow rate of each which is the same or different but is within the range from 0.2 to 0.3 kg/min where jet nozzles of 0.02 inch internal diameter are employed to create high turbulence at their point of impact to achieve high intensity micromixing of each stream prior to nucleation, to produce a slurry of crystals of aripiprazole monohydrate; and (d) recovering crystals of aripiprazole monohydrate of desired small particle size and narrow particle size distribution, wherein 10% of the crystals of aripiprazole monohydrate produced have a particle size of less than 10 microns, 50 % of the crystals of aripiprazole monohydrate produced have a particle size of less than 35 microns and 95 % of the crystals of aripiprazole monohydrate produced have a particle size of less than 100 microns, and wherein said crystals of aripiprazole monohydrate have a mean particle size within the range from 25 microns to 100 microns.