Third-instar larvae of the goldenrod gall fly Eurosta solidaginis (Diptera: Tephritidae) from populations in northern North America transition from freeze-susceptible to freeze-tolerant just prior to the onset of winter. While studies have documented the accumulation of carbohydrate cryoprotectants during this transition, protein cryoprotectants common to other freeze-tolerant species have not been reported in the gall fly. Using larvae collected from a population in Madison County, NY, which changes from freeze-susceptible to freeze-tolerant in early October, we assayed for the presence of factors that could preserve the catalytic activity of the cold-labile enzyme, rabbit muscle lactate dehydrogenase. Freezing this enzyme with a heat-stable, hydrophilic fraction derived from homogenates of both freeze-tolerant larvae and those in the process of becoming freeze-tolerant preserved between 70% and 80% of this enzyme's activity. Neither a comparable solution of bovine serum albumin nor the naturally-occurring carbohydrates (glycerol, sorbitol, or trehalose) conferred this level of cryoprotection. The putative cryoprotective protein from gall fly larvae did not bind to a weak anion exchanger, implying that its character may be cationic.