The mycoparasite Trichoderma harzianum, a biocontrol agent, forms coils and other mycoparasitic structures upon contact with host fungi. A biomimetic system consisting of nylon fibres was used to test the involvement of signal transduction pathways in the induction of coils. Two activators of G protein-mediated signal transduction induced coiling of hyphae around nylon fibres. The peptide toxin mastoparan increased coiling more than two-fold in comparison with controls. The activator fluoroaluminate (A1F4−) had a similar effect, whereas aluminium ions alone were ineffective. cAMP increased coiling about three-fold. Although the two G protein activators, mastoparan and fluoroaluminate, have very different modes of action, they share the Gα subunit as a target. We also found that two T. harzianum isolates differ in their extent of coiling around nylon fibres in the absence of effectors. The results reported here demonstrate that the biomimetic system can be used to study the biochemistry of coil induction, and will be a valuable assay to aid in the genetic manipulation of this pathway. It is proposed that a signal for mycoparasitic behaviour from the host cell surface is transduced by heterotrimeric G protein(s) and mediated by cAMP.