In order to search for potential molluscicidal synergists, eighteen O,O'-dialkyl-O''-(5-substituted-3-benzothienglyoxylonitrile oximino) phosphates and thiophosphates were synthesized. Preliminary biological screening showed that compounds I2,3,7,11,12 combined with sodium pentachlorophenate exhibited significant molluscicidal synergism against snails (Oncomelanis hupensis).