A liquid larval diet and its rearing system for Bactrocera cucurbitae Coquillett fruit fly production were developed. The diet was composed of brewer’s yeast, sugar, antifungal agents (sodium benzoate and nipagen), citric acid, and distilled water. Sponge cloth placed in rearing trays was used as a support substrate for larvae, alleviating the need for the traditional (mill feed) bulking agent. Larval rearing of B. cucurbitae on this diet resulted in ≈20% less pupal production and ≈10% lighter pupal weight than from the control diet, whereas pupal density, adult emergence, adult fliers, and egg hatch showed no significant discrepancies. Pupal recovery increased with yeast concentrations up to 14.2%. Benefits derived from a liquid diet include reduction in postrearing waste, alleviation of (pesticide-free) bulking agent, and reduction in diet ingredient storage and labor. These benefits must be weighed against any reductions in production and size when large-scale mass rearing of fruit flies for use in sterile insect release programs are evaluated.