Aims: For Ophiostoma (Ceratocystis) ulmi, the ability to undergo morphological change is a crucial factor for its virulence. To gain an understanding of quorum-sensing activity in O.ulmi as it relates to yeast-mycelium dimorphism control, this study examines the effects of branched-chain amino acids as well as their fusel alcohols and fusel acids as quorum sensing molecules. Methods and Results: In a defined medium containing glucose, proline and salts, O.ulmi grew as yeasts when the culture was inoculated with a high density of spores (2 x 107 CFU ml-1) and as mycelia when inoculated with a low spore density (4 x 105 CFU ml-1). The cultures displaying yeast morphology secreted a quorum-sensing factor that shifted the morphology from mycelia to yeast. This quorum-sensing molecule was lipophilic and extractable by organic solvents from the spent medium. Using GC/MS analysis, it was determined that the major compound in the extract was 2-methyl-1-butanol. A similar effect was observed when the branched-chain amino acids (fusel alcohol precursors) were used as the nitrogen source. E, E-farnesol had no effect on the morphology of O.ulmi. Conclusions: Addition of the branched-chain amino acids or one of the compounds detected in the spent medium, 2-methyl-1-butanol or 4-hydroxyphenylacetic acid, or methylvaleric acid, decreased germ tube formation by more than 50%, thus demonstrating a quorum sensing molecule behaviour in O.ulmi cultures. Significance and impact of the study: This study presents advances in the investigation of dimorphism in O.ulmi, complementing the existing scientific basis, for studying, understanding and controlling this phenomenon.