Conidial anastomosis tubes (CATs) can be recognized in 73 species of filamentous fungi covering 21 genera, and develop in culture and in host-pathogen systems. They have been shown to be morphologically and physiologically distinct from germ tubes in Colletotrichum and Neurospora, and under separate genetic control in Neurospora. CATs are short, thin, usually unbranched and arise from conidia or germ tubes. Their formation is conidium-density dependent, and CATs grow towards each other. MAP kinase mutants of Neurospora are blocked in CAT induction. Nuclei pass through fused CATs and are potential agents of gene exchange between individuals of the same and different species. CAT fusion may also serve to improve the chances of colony establishment.