By means of conventional light microscopical and counting procedures, estimations have been made of the average number and size of ovarian balls from unfertilized and fertilized female Moniliformis dubius. The observations were made on worms recovered from experimental primary infections lasting 1, 2, 3, 4, 5, 8, 11, 14, 17 and 20 weeks in male rats. The number of ovarian balls rises to about 6300 per worm, with the maximum being attained more rapidly in unfertilized than in fertilized females. On average more ovarian balls were present in unfertilized than in fertilized females, but a marked decline in number occurred in both groups of worms towards the end of the experimental period. Similarly, on the basis of the area within the elliptical profiles of fixed ovarian balls, it appeared that those from unfertilized females were generally larger on average than those from fertilized worms.