The co-toxicities of tea saponin mixed with pesticides Bt, rotenone and dimethoae were testedrespectively. Results showed that tea saponin had significant synergistic actions on these pesticides. Afteradded tea saponin at concentrations of 1 000 and 2 000 mg/L into pesticide dilutions, the co-toxicity co-efficients of Bt admixtures on the third and fourth instar larvae of diamondback moth Plutella xylostella(L.) were 168. 10, 260. 93 and 408. 15, 543. 68, respectively, while the LC50 s of rotenone anddimethoate to turnip aphid Lipaphis erysimi (Kaltenbach) were reduced by 34. 88%, 58. 10% and 18.1 3 %, 53. 70% respectively.