In pea (Pisum sativum) up to 50 nodulation mutants are known, several of which are affected in the early steps of the symbiotic interaction with Rhizobium sp. bacteria. Here we describe the role of the sym2 gene in nodulation (Nod) factor perception. Our experiments show that the sym2A allele from the wild pea variety Afghanistan confers an arrest in infection-thread growth if the Rhizobium leguminosarum bv viciae strain does not produce Nod factors with a NodX-mediated acetylation at their reducing end. Since the induction of the early nodulin gene ENOD12 in the epidermis and the formation of a nodule primordium in the inner cortex were not affected, we conclude that more than one Nod factor-perception mechanism is active. Furthermore, we show that sym2A-mediated control of infection-thread growth was affected by the bacterial nodulation gene nodO.