Crude root extracts from Mexican avocado trees (Persea americana) were screened for antioomycete activity against Phytophthora cinnamomi. Fortyeight accessions from Mexican avocado trees were selected with potential resistance to P. cinnamomi according to environmental and site descriptors. Crude root extracts from these accessions were obtained and tested in vitro against the oomycete P. cinnamomi. Seven crude root extracts inhibited mycelial growth (>50%) and only root extracts from accessions 765-01 and 773-01 showed 100% of inhibition. Extracts from accessions 765-01 and 773-01 were analysed by preparative thin-layer chromatography and six fractions were detected under UV light. In both extracts, fractions IV (R f 0.85) and V (R f 0.9) showed in vitro inhibition (100%) against mycelial growth of P. cinnamomi. Fraction V was subjected to GC-MS analysis and stigmastan-3,5-diene (C 29 H 48 ) was identified as the major compound. In vitro assay showed that stigmastan-3,5-diene (100 ppm) inhibited the mycelial growth of P. cinnamomi. The constitutive presence of this compound in avocado roots offers possibilities to identify and to select potentially resistant plants to P. cinnamomi.