An ultrastructural study of the subcellular events during the initial stages of infection of nematodes by conidia of the endonematophagous fungus Drechmeria coniospora showed that adhesion to the nematode cuticle occurred by means of an adhesive knob at the distal end of the spore. An appressorium, also covered by an adhesive layer, was formed from the adhesive knob before penetration. The penetration tube arose from the appressorium where it contacted the cuticle. Acid phosphatase activity was detected at the site of penetration prior to penetration. It is suggested that penetration of the nematode was due to the combined action of enzymic softening of the cuticle and mechanical force.