Abstract: A bulk yarn formed from a plurality of multi-filament yarns having alternating compact and open segments. The compact segments include first and second plaited portions which are twisted in opposite directions. The open segments of the yarn are defined by loosely bundled multi-filament yarn. The apparatus for bulking the yarn includes an elongated housing having a longitudinal bore extending therethrough. Pressurized air passes through diametrically opposed passages for contacting the yarn as it passes through the bore for producing the bulking effect thereon.