Although macromolecular purity is thought to be essential for the growth of flawless protein crystals, only a few studies have investigated how contaminants alter the crystallization process and crystal quality. Likewise, the outcome of a crystallization process may vary with the crystallization method. Here, it is reported how these two variables affect the crystallogenesis of aspartyl-tRNA synthetase from the eubacterium Thermus thermophilus. This homodimeric enzyme (Mr=130,000) possesses a multi-domain architecture and crystallizes either in a monoclinic or an orthorhombic habit. Minute amounts of protein impurities alter to a different extent the growth of each crystal form. The best synthetase crystals are only obtained when the crystallizing solution is either enclosed in capillaries or immobilized in agarose gel. In these two environments convection is reduced with regard to that existing in an unconstrained solution.