SUMMARYThirteenn‐alkyl guanidine acetates were prepared and tested against conidia ofVenturia inaequalis.Maximum fungistatic activity was associated with a chain length of 12–15 carbon atoms. Selected members of the series were also tested against conidia ofNectria galligenaand in a greenhouse assay against apple mildew, caused byPodosphaera leucotricha.These fungi proved in general more tolerant of the guanidines than wasV. inaequalis, but were markedly checked by then‐dodecyl homologue (dodine acetate).For comparison, tenn‐alkylamine hydrochlorides were also tested againstV. inaequalis, and found to be generally les