Abstract.1. Species of the genusCordyceps(Ascomycotina; Clavicipitales) are the commonest fungi encountered on arthropods in tropical forests.2. The asexual states ofCordycepsmay occur in conjunction with or separately from the perfect state and are classified in the generaHirsutella, Hymenostilbe, Nomuraea, PaecilomycesandVerticilliumof the imperfect fungi (Deuteromycotina; Moniliales).3. MostCordycepsspecies have a restricted host range and this rigid host specificity, at the generic or tribal level, is especially evident in ant‐fungal associations.4. Pathogenicity was not tested but circumstantial evidence is presented which supports the view thatCordycepsspecies are primary pathogens of arthropods.5. Observations of living, infected ants suggest that behavioural patterns are radically altered, possibly altruistically orientated.6. It is thought that entomogenous fungi are involved in the regulation of arthropod populations and may help to maintain stability in tropical forest ecosystems.7. The value of these organisms to man may be in the metabolites they produce rather than in their direct usage as biological control agents of insect pest