The theory necessary for the optimization of second-harmonic (SHG) efficiency in biaxial crystals is developed and applied to the molecular crystal (-)2--(methylbenzylamino)-5-nitropyridine (MBANP). It is shown that, for the range of crystal lengths and beam walk-off angles available, the criteria for the selection of the parameters and for optimization of the second-harmonic signal are the same in biaxial and uniaxial crystals. The maximal possible SHG figure of merit, , is found to be for a 50 mm long crystal.