During the course of carrying out studies on the role of intestinal immunity in blocking the migration of larval Ascaris suum in the pig, it was discovered that the prevailing understanding of larval penetration sites was at variance with our observations. Therefore, a detailed investigation of the migration of A. suum L2 larvae through the intestine was initiated. The results demonstrate that the L2 larvae invade almost exclusively the wall of the pig cecum and colon and not the small intestine as is generally believed. The larvae were recovered from the mucosa of the cecum and colon as early as 3 hr postinoculation (PI) with infective eggs and were recovered from the liver by 6 hr PI. The maximal recovery of larvae (total larvae and larval/g of mucosa) from the intestinal mucosa occurred between 6 and 12 hr PI; by 24 hr PI, virtually all of the larvae had disappeared from the mucosa. These observations correct a common misunderstanding of this aspect of the life cycle of A. suum in the pig, and they raise 2 issues related to the biology and pathogenesis of Ascaris in humans. What is the actual migratory and development behavior of Ascaris lumbricoides and A. suum in humans and the potential risk for liver lesions? Most authors, in describing the life cycle of A. lumbricoides, either ignore or discount a possible obligatory liver stage of development, and, consequently, the potential for lesion formation similar to that which occurs in pigs infected with A. suum. This issue takes on added importance with the growing evidence that A. suum is an important zoonoses.