The e โ ects of adjuvants on the performance of a dispersible concentrate formulation (DCI) of a novel powdery mildew fungicide, 1-(4-chlorobenzyl)-4-phenylpiperidine (I) were investigated. The method involved assessment, under glasshouse conditions, of the therapeutic (curative) control of infections of powdery mildew (Erysiphe graminis DC f.sp. hordei Marchal) on barley (Hordeum vulgare, L.) eight to nine days after spray application to plants that had been inoculated one day prior to spraying. The results from the รrst trial showed that marked improvements (Dรve-fold) in the performance of DCI could be obtained by the spray tank addition of some types of surfactant adjuvants and a series of further trials investigating a wide range of adjuvants was conducted. Nonylphenol, alkylamine and alcohol ethoxylates varying in mean ethylene oxide content between 5 and 20 moles were highly e โ ective. There were indications that optimum performance enhancements were obtained with these surfactants containing between 5 and 10 moles ethylene oxide. Lower enhancements, sometimes only marginal, were obtained from trisiloxane, phosphate ester, propylene oxide, alkylamine-propylene oxide and castor oil ethoxylates and also alkyl polyglucoside biosurfactants. Negligible adjuvant enhancements were observed with emulsiรable paraffinic/naphthenic and rape seed oils, though slightly better enhancements were seen with an emulsiรable transmethylated rape seed oil and, interestingly, larger enhancements with an emulsiรable lipophilic alcohol. A รnal trial identiรed the alcohol ethoxylate, "Dobanolร 91-6, as the most e โ ective adjuvant and that its optimum application rate under glasshouse conditions was 250 g ha~1. This information will be used to guide the design of รeld trials.