To observe the biological characteristics of the juvenile of Paragonimus westermani.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 degrees C to 8 degrees C and invasiveness of juveniles to new hosts were detected by host transfer.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.The juvenile of P. westermani is an infective stage, having strong vitality and invasiveness.