An efficient one-step transformation method for the dimorphic yeast Yarrowia lipolytica is described. Using cells grown overnight on agar plates, the whole process is carried out within 1 h. The transformant clones could be recovered on selective plates as early as 36-48 h after plating, The efficiency was better than 10(5) transformants/mu g replicative plasmid DNA. Effects of cell density, dithiothreitol, heat shock, poly(ethylene glycol) 4000 concentration and the wetness of selective plates were investigated.