Abstract: 　AIM: To observe the biological characteristics of the juvenile of Paragonimus westermani. METHODS: Kunmin strain mice and Wistar rats inoculated orally with 100-150 metacercariae of Paragonimus westermani were autopsied 10-30 days and 80-580 days after infection, respectively. Life-span, survival time in dead hosts or physiological saline at 5℃ to 8 ℃ and invasiveness of juveniles to new hosts were detected by host transfer. RESULTS: All the worms recovered from the mice and rats were stunted juveniles. The worm detection rates in the mice and rats were 19.3% and 22.2%-37.5%, respectively. The life-span of the juveniles was rather long, being not limited by the life-span of a host. Host transfer could prolong longevity and preserve invasiveness. The juveniles recovered from dead mice still possess strong vitality and invasiveness. CONCLUSION: The juvenile of P.westermani is an infective stage, having strong vitality and invasiveness.\;

Key words: Paragonimus westermani, juvenile, life-span, invasiveness, mouse, rat.