Abstract: "Paramecium bursaria-Chlorella sp." system,a kind of ideal symbiotic system,was selected to research host-symbiont interaction.In order to discover the difference of genomic DNA of host and symbiont,genetic variation of the chlorellae-bearing and chlorellae-free Paramecium bursaria was detected by randomly amplified polymorphic DNA (RAPD) markers.Genetic variation from 12 polymorphic fragments amplified with primers S174,S750,S769,S755,S103 in both strains of Paramecium bursaria exited,though they shared the same fragments amplified with primers S401,S147,S786,S759,S086,S763,S155.It is implied that some changes in genomic DNA of P.bursaria have occurred owing to symbiotic functions.Also,it is shown that there are the similarity between the amplified product of P.bursaria and symbiotic chlorellae.