Abstract: Methods for preparing crystalline L-glufosinate ammonium monohydrate are disclosed. The methods include forming a mixture comprising water, a water-miscible organic solvent, ammonium hydroxide, and a glufosinate starting material containing L-glufosinate ammonium and D-glufosinate ammonium. L-Glufosinate ammonium monohydrate is crystallized and separated from the mixture, providing L-glufosinate ammonium monohydrate Form B. Compositions and methods employing the crystalline L-glufosinate ammonium monohydrate are also described.