Abstract: Twenty compounds of O,O'-dialkyl-O"-(substituted phenylglyoxylonitrileoximino) phosphates and thiophosphates were prepared.The screening tests showed that compoundsV_4,7,12,18 combined with niclosamide exhibited significant molluscicidal synergism against snails(Oncomelania hupensis hupensis).The LC₅₀ of niclosamide alone and V₁₂ combined with niclosamide(1:1)against snail was 0.0820 mg·L^-1 and 0.0215 mg·L^-1,respectively. The molluscicidalactivity of the latter was 3.81 times that of niclosamide alone.The compounds V_{2,7, 10,13} appeared tohave high molluscicidal activity.