Introduction The ability to substitute single homologous pairs of chromosomes from a donor variety for their homologues in a recipient variety provides a ready means of analysing the genotype of Triticum aestiwzmz (Sears, 1953; Kuspira and Unrau, 1957; Law, 1966a). The substitution of single chromosomes, however. can reflect onlv the variation which exists between com~lete chromoJ 1 somes; it cannot indicate the number and the type of loci which are ultimately responsible for this variation. T o undertake this kind of analysis, further crossing procedures must be employed in which the chromosomal units provided by a substitution series are broken down into components which are closer to the conventional units of inheritance, the genes. A technique with this aim in view was suggested initially by Unrau (1958) and a modification of this was used bv Law (1966b) to locate some of the factors J controlling the time to ear emergence on chromosome 7B. In this paper an account will be given of the location of other genetic factors, notably those conferring resistance and susceptibility to Erysiphe gmnzizis, on the same chromosome. Materials and Methods The genotypes used in the present investigation were identical to those used in the previous study (Law, 1966b). This involved the derivatives of three types of cross in which monosomics for chromosome 7B of the variety Chinese Spring (CS) were used as female parents in each case. The three genotypes used as the pollen parents were:(i) A substitution line in which chromosome 7B of the variety Hope replaced its homologue in the variety Chinese Spring, i.e. CS (Hope 7B), (ii) The recipient variety, Chinese Spring, i.e. CS, and (iii) The hybrid between (i) and (ii) in which chromosome 7B was heterozygous but the remaining chromosomes homozygous. In the case of the crosses in which CS (Hope 7B) and CS were the pollen parents, the monosomic derivatives must be hemizygous for chromosome 7B of Hope and CS, respectively. Therefore the monosomic derivatives from these two crosses, when combined, can be regarded as a sample of the two nonrecombinant 7B chromosomes. Such a sample mav be referred to convenientlv as the parental products. On the other' hand: the monosomic derivativis resulting from cross (iii), designated in this case as the F , products, must provide a sample of both non-recombinant and recombinant 7B chromosomes and as such shoulh show the segregation of any alleles which distinguish chromosome 7B of Hope from its homoloeue in CS. I U This technique has its greatest application where quantitative characters are being studied, since by statistically comparing the distributions of the