High density polyethylene (HDPE) syntactic foams containing fly ash cenospheres as the hollow filler are fabricated using an industrial scale injection molding machine and studied for their dynamic mechanical behavior. Syntactic foams using thermoset matrix materials and engineered glass hollow particles have long been used as buoyancy devices and thermal insulation in the marine sector and as a lightweight sandwich core in the aerospace industry. This class of materials is attractive because of high mechanical properties in compression, tailorable density, and improved thermal properties. The constituents are used in as-received condition, without surface treatments. These lightweight composites can be highly beneficial in developing consumer goods by reducing consumption of HDPE. Syntactic foams are produced containing 20, 40, and 60% cenospheres by weight. A temperature sweep from 35-130°C and a frequency sweep from 1-100 Hz are conducted on the fabricated syntactic foams. At all temperatures, syntactic foams show higher storage and loss moduli and lower damping than neat HDPE. Syntactic foams with 60 wt.% cenospheres show only a small increase in properties compared to those with 40 vol.% due to particle breakage during processing. However, high particle loading has the benefit of reducing consumption of HDPE. The time-temperature superposition principle is used to extend the frequency response to cover the range 10-2-106 Hz.