The adult tapeworm, Hymenolepis diminuta, living in the lumen of the intestine of its normal rat host was shown to sensitize the intestine. The growth of worms in a secondary infection, given 8, 23, or 41 days after the primary infection had been removed by anthelmintic treatment, was 0.5, 3, and 25%, respectively, of that in the control worms. Thus, there is a strong initial resistance to a challenge infection, but this resistance weakens fifty-fold between days 8 and 41 postinfection. Even primary infections of five worms, which would have survived indefinitely if not expelled by anthelmintics, depressed the growth by over 80% of secondary infections given 8 days later. Rats given a light primary infection manifested their sensitivity only by depressing the growth of challenge worms, but in strongly sensitized rats, worm growth was stunted severely (or the worms destrobilated), partial worm expulsion occurred, and the remaining worms migrated to a more anterior position in the intestine. We suggest that the rapid waning of memory indicates that the protective mechanism evoked in the intestine has as its primary biological function the limitation of the number of worms that can accumulate in the intestine in concurrent infections, not the prevention of reinfection after the loss of worms from previous infections.