The parasite-induced weight gain in animals is an unusual phenomenon which involves a complex host–parasite relationship. Trichinella pseudospiralis exhibits such a phenomenon in mice. Eighty Swiss mice were orally infected with 600 T. pseudospiralis or Trichinella spiralis muscle larvae. The total body weight of the mice was recorded at the time of infection and at 2 and 10 months after, and compared with an uninfected control group. Trichinella pseudospiralis muscle larvae induced an abnormal weight gain. This weight increase is statistically different from the other two groups of mice after 2 and 10 months postinfection. The results indicate the existence of a different type of host–parasite relationship between the two parasites. Some speculative ideas are advanced to explain the rapid growth effect of T. pseudospiralis on its host.