Materials with high thermal conductivity (kappa) are of technological importance and fundamental interest. We grew cubic boron nitride (cBN) crystals with controlled abundance of boron isotopes and measured greater than 1600 watts per meter-kelvin at room temperature in samples with enriched B-10 or B-11. In comparison, we found that the isotope enhancement of kappa is considerably lower for boron phosphide and boron arsenide as the identical isotopic mass disorder becomes increasingly invisible to phonons. The ultrahigh kappa in conjunction with its wide bandgap (6.2 electron volts) makes cBN a promising material for microelectronics thermal management, high-power electronics, and optoelectronics applications.