Abstract: At least one crystalline hydrate of (S)—[N-3-(3?-fluoro-4?-(4?-phenyl piperazinyl))phenyl-2-oxo-5-oxazolidinyl]methyl acetamide, such as those with the following formula: wherein y is a number ranging from 1/12 to 1. Also provided are methods for the preparation of such crystalline hydrates, pharmaceutical compositions comprising such crystalline hydrates, and methods for their uses.