When a 70-min exsheathing process and two-step roller culture system were used, infective larvae of Ostertagia ostertagi developed in large numbers to mature adult males and females in 28 and 29 days, respectively. Females laid fertile eggs in the 2- to 16-cell stages from day 34 on. Some of the expelled eggs developed to the morula stage, in the culture fluid, by day 52. Yields of mature adults and eggs were best when cultures were started with 100,000 larvae in 100 ml of medium. The optimal culture system produced from 20% to 35% mature adults, from days 28 through 52.