The formation of interconnected hyphal networks is central to the organisation and functioning of the filamentous fungal colony. It is brought about by the fusion of specialised hyphae during colony initiation and mature colony development. These hyphae are normally genetically identical, and hence this process is termed hyphal self-fusion. The conidial anastomosis tube (CAT) functions in forming networks of conidial germlings during colony initiation. This hyphal type in Neurospora crassa is being used as a model for studies on hyphal self-signalling and self-fusion in filamentous fungi. Extraordinary new insights into the process of self-signalling that occurs during CAT self-fusion have recently been revealed by live-cell imaging of genetically engineered strains of N. crassa. A novel form of signalling involving the oscillatory recruitment of signal proteins to CAT tips that are communicating and growing towards each other has been observed. This 'ping-pong' mechanism operates over a very short time scale and comparisons with non-self-signalling during yeast cell mating indicate that this mechanism probably does not involve transcriptional regulation. It is proposed that this mechanism has evolved to increase the efficiency of fusion between genetically identical cells that are non-motile.