The mutant E135 (sym 13) of pea (Pisum sativum L.) forms a normal number of small white nodules that contain bacteroids, but these bacteroids lack nitrogenase activity. To evaluate the effects of the sym 13 gene on the expression of nodulin genes, cDNA clones for nodulins were isolated from pea nodules and the expression of nodulin genes in ineffective E135 nodules was compared with that in nitrogen-fixing nodules on the wild-type parent, cv. Sparkle. Nineteen cDNA clones for nodulins, including ENOD2 and cDNAs for two distinct leghemoglobins (Lbs), were isolated from Sparkle nodules by a subtractive hybridization procedure. All the nodulin genes examined were expressed in nodules on both E135 and Sparkle plants. However, the level of expression of seven genes, one of which was an Lb gene that corresponded to PsN5, was significantly lower in E135 nodules. The levels of Lb apo-proteins, with the exception of Lb-III and Lb-IV, in E135 nodules resembled those in Sparkle nodules, but the level of heme in E135 nodules was lower than that in Sparkle nodules. Although the expression of the two Lb genes that corresponded to PsN5 and PsN120 in E135 nodules was slightly depressed by exogenous ammonia, the level of the PsN5 transcript was still lower than the control level in Sparkle nodules. Our results indicate that the plant gene sym 13 does not influence the induction of nodulin genes but does influence the level of the expression of some genes, one of which is a gene for Lb, as well as the level of heme.