1. When adult Dirofilaria immitis were incubated up to 60 min in a medium containing physiological concentrations of glycerol and glucose, the mean amount of free glycerol present did not change significantly. 2. [14C]2-glycerol disappeared from media in which 1 g heartworms were incubated at a linear rate of about 0.7 mumol/hr/g during the first hour. 3. 85-95% of the radioactivity in the Folch extract remained in the aqueous phase. 4. Phosphoglycerides and diacylglycerols accounted for better than 90% of the 14C in the lipids. 5. The rate of glycerol conversion to lactate was only about 0.02% of the rate of glucose conversion under experimental conditions. 6. The data indicate that, although dog heartworms do utilize glycerol as a substrate in glycolysis, it is probably of more importance in lipid synthesis.