Nonnodulating (Nod-) legumes are valuable reference plants for studies on quantification of biological nitrogen fixation. Several such Nod- chickpea genotypes have been identified at ICRISAT and elsewhere, and the inheritance of the Nod- trait worked out in some of them. The present study was undertaken to determine the genetics of nonnodulation in the newly identified Nod- mutants, and the genetic relationship among these and the ones reported earlier. Five new Nod- mutants were crossed with their Nod+ parents. These, and two previously reported Nod- mutants were also crossed among themselves. Parents and the F1, F2, and backcross progenies of the above crosses were inoculated with a mixture of rhizobial strains, and tested for the presence or absence of root nodules in the greenhouse. A single recessivegene was found to control nonnodulation in all but two mutants. Similarly, genetic complementation to produce nodules was observed in most mutant '' mutant crosses. Accordingly, six parents were classified into four allelic groups, and two new nonnodulation genes were identified. Further studies have been proposed to resolve some of the anomalies in segregation ratios. This paper also reports breakdown of the nonnodulation trait in two of the seven Nod- lines, PM 233 and ICC 5003 M
