Abstract Infection of melon plants by Colletotrichum lagenarium induces the production of five mRNA species (1·4, 1·65, 2·15, 3·2 and 4·5 kB) which hybridize to a genomic clone of hydroxyproline rich glycoproteins (HRGP) from carrot. Induction takes place 3 to 4 days after inoculation, which is consistent with the time course of accumulation of HRGP in the cell wall upon infection. mRNAs are expressed in leaves, stems, and roots. All transcripts, except the 4·5 kb species, are present among the cytosine-rich RNA isolated by affinity chromatography on an oligo(dG) cellulose column. Treatment of melon plants with ethylene leads to early induction of the 1·4 and 1·65 species and, to a lesser extent, the other three transcripts.