A collapsible mixing drum is provided in which two similarly contoured, frustoconical members, one of which is open and the other closed at the small end, and both of which are provided with a plurality of circumferentially spaced and inwardly offset stiffening and mixing ribs, terminating in radial wall portions at the large ends of the frustoconical members, the radial wall portions having aligned apertures to receive axially oriented fasteners, are detachably joined together through clamping engagement with an annular ring member having on opposed radial surfaces thereof aligned grooves for sealably receiving circumferential edges of the large ends of the frustoconical members, and the outer surface of the ring member being contoured to provide a circumferential groove, gear teeth, or a combination thereof to facilitate rotational support and/or rotational driving of the assembled drum.The frustoconical members, and the axially extended ring if present, are preferably fashioned from polyethylene or other plastic material providing a durable light-weight structure from which hardened cement can readily be removed; and the collapsible nature of the drum provides space saving advantages in the manufacture, distribution and storage of mixing devices in which the drum is used.