InP crystals were grown by the horizontal Bridgman method under controlled ambient phosphorus vapor pressure. The relationship between melt composition and crystal defects was investigated. Crystals grown under lower phosphorus vapor pressure have an In-rich composition. Dislocation densities (etch pit densities: EPD) and FWHM of X-ray diffraction minimized at a specific phosphorus pressure between 20 and 29 atm. From the reciprocal space mapping measurements, it is also shown that crystals grown under high or low phosphorus vapor pressure consist of mosaic domains surrounded by tilted boundaries.