InP single crystals were grown by the horizontal Bridgman method under controlled ambient phosphorus vapor pressure. The relationship between melt composition and crystal defects was investigated. The InP grown under phosphorus vapor pressure of 25 atm has EPD levels of 10(3) cm(-2) through the ingot. In the InP grown at 12 or 29 atm, the EPD increased more than 2 orders at the tail end. X-ray reciprocal maps indicated that the crystals consisted of many domains with tilt boundaries at a distance from the seeding point. The deviation of the lattice parameter measured by the X-ray diffraction angle indicated that the composition changed gradually. The composition near the seeding point was nearly stoichiometry under the influence of the composition of source polycrystalline, because a finite time was required to transition of phosphorus. Optimum phosphorus vapor pressure is necessary to obtain InP with stoichiometric composition and low defect densities.