Phosphatidic acid (PA) has only recently been identified as an important eukaryotic lipid-signalling molecule. In plants, PA formation is triggered by various biotic and abiotic stresses, including wounding, pathogen attack, drought, salinity, cold, and freezing. However, few molecular targets of PA have been identified so far. One of the best characterized is Raf-1, a mammalian MAPKKK. Arabidopsis thaliana CTR1 (constitutive triple response 1) is one of the plant homologues of Raf-1 and functions as a negative regulator of the ethylene signalling pathway. Here, it is shown that PA binds CTR1 and inhibits its kinase activity. Using different PA-binding assays, the kinase domain of CTR1 (CTR1-K) was found to bind PA directly. Addition of PA resulted in almost complete inhibition of CTR1 kinase activity and disrupted the intramolecular interaction between CTR1-K and the CTR1 N-terminal regulatory domain. Additionally, PA blocked the interaction of CTR1 with ETR1, one of the ethylene receptors. The basic amino acid motif shown to be required for PA binding in Raf-1 is conserved in CTR1-K. However, mutations in this motif did not affect either PA-binding or PA-dependent inhibition of CTR1 activity. Subsequent deletion analysis of CTR1's kinase domain revealed a novel PA-binding region at the C-terminus of the kinase.