Cerato-ulmin (CU) is a fungal toxin class II hydrophobin, involved in Dutch elm disease. The formation of hydrophobin films at the air-water interface is a key mechanism which plays a role of paramount importance at different stages of the fungal development. We present a study on the precursor stages of growth towards the self-assembly aggregation film of CU. Atomic force microscopy images of CU dropped on mica substrates indicate that the system self-organizes in almost one-dimensional pearl-necklace-like chains, which subsequently collapse and possibly merge to form extended and rather compact planar films. We propose and verify a simple model to describe the self-aggregation mechanism in terms of progressive thickening of the pearl chains due to the successive merging and collapse of the elementary constitutive units.