The 3rd stage larvae of Metastrongylus apri were successfully cultured in vitro to the nearly maturity adult worms which possessed the sperm and immature eggs, though the worms were smaller in size than those grown in vivo. The most excellent medium for the development was relatively simple and consisted of chemically defined NCTC 109 supplemented with 20% calf serum, penicillin (200 unit/ml) and streptomycin (100 micrograms/ml). Standing culture with a larval concentration of 100 to 200 worms per tube containing 2 ml medium and wih a gas phase of 5% CO2 in air at 37 to 28 degrees c was found to be sufficient twice a week using aseptic techniques. Under these conditions a relatively uniform development was obtained, attaining 85 +/- 6.7% of larvae to the 5th stage.