Polystyrene (PS) is generally considered to be durable and resistant to biodegradation. Mealworms (the larvae of Tenebrio molitor Linnaeus) from different sources chew and eat Styrofoam, a common PS product. The Styrofoam was efficiently degraded in the larval gut within a retention time of less than 24 h. Fed with Styrofoam as the sole diet, the larvae lived as well as those fed with a normal diet (bran) over a period of one month. The analysis of fecula egested from Styrofoam-feeding larvae, using gel permeation chromatography (GPC), solid state 13C cross polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectroscopy and thermogravimetric Fourier transform infrared (TG-FTIR) spectroscopy, substantiated that cleavage/depolymerization of long-chain PS molecules and the formation of depolymerized metabolites occurred in the larval gut. Within a 16-day test period, 47.7% of the ingested Styrofoam carbon was converted into CO2, and the residue (ca. 49.2%) was egested as fecula with a limited fraction incorporated into biomass (ca. 0.5%). Tests with α13C- or β13C-labeled PS confirmed that the 13C-labeled PS was mineralized to 13CO2 and incorporated into lipids. The discovery of the rapid biodegradation of PS in the larval gut reveals a new fate for plastic waste in the environment.