SUMMARYThe systemic anti‐mildew activity shown by homologous series of alkoxy‐, alkylamino‐ and alkylthio‐trichloroethyl formamides when applied to the roots of wheat or cucumber seedlings has been measured. In general the thio compounds were much less active than their oxy and amino analogues. The alkyl grouping giving most activity varied from one series to another and with test method. In cucumber plants the most prolonged protection against mildew was given by thesec‐butoxy andsec‐butylamino compounds. When inoculated disks of cucumber leaf tissue were floated upon solutions of the compounds, smaller differences in anti‐mildew activity were found than when application was made thro