The effect of feeding different numbers of Toxocara canis eggs on patent intestinal infections was studied in 45 ascarid-naive 62--64-day-old pups and 6 adult dogs. Twenty-four of 25 pups fed 10 to 1,000 eggs developed patent intestinal infections, whereas none of the 20 pups fed 10,000 eggs developed patent intestinal infection. Three of 6 presumably ascarid-naive dogs, 7, 10, and 52 months old fed 100 eggs each, developed intestinal infections.