We studied changes in gene expression during the establishment of vesicular-arbuscular (VA) mycorrhizal symbiosis. Polypeptides were obtained by in vitro translation of total root RNA extracted from VA-colonized and noncolonized root-tissue of onion (Allium cepa L. cv. Babosa), and resolved by two-dimensional polyacrylamide gel electrophoresis. VA mycorrhization led to a specific appearance of eight new polypeptides, and the disappearance of seven polypeptides in VA-colonized root. Our findings indicate that gene expression is altered in response to morphological and physiological changes resulting from the establishment of VA mycorrhizas.