Cultures of Cooperia punctata, a nematode parasitic in cattle, were studied in Ae medium at a pH of 7.2 to 7.3 under air. In fourth and fifth stages of development, they absorbed or otherwise took into their free pool and tissues, carbon derived from 14C-labeled sodium propionate and converted, by pathways not elucidated, a portion of this carbon into protein and lipid fractions. Thirty minutes postincubation in balanced slat solution was adequate to reduce the amount of isotope in the nematode gut to a constant level. Activity from specifically-labeled 14C-propionate was recovered (as glucose pentaacetate) from worm glucose and from CO2 evolved from cultures consisting of L4 and adult stages. Use of propionate by these worms, for whatever metabolic purpose, would result in depriving the ruminant host of some of its necessary glucogenic precursors, and could account for a specific pathogenic mechanism attendant to heavy infections with this parasite.