To analyse the genetic aspects of C₃ and C₄ photosynthetic systems, the photosynthetic charcacteristics of the progenies derived from a backcross, (Atriplex rosea (C₄, 2 n=18, 2x)xA.patula (C₃, 2n=36, 4x))xA, rosea, were investigated. Out of three plants derived from the backcross, two showed C4-Iike traits based on the C0₂ compensation point and δ¹³C values as well as enzyme analysis and leaf anatomy. Another plant showed C₃-like characteristics. The descendants of these plants were segregated into three discrete morphofogical types ; p-type plants which were similar to A. patula ; d-type plants which were dwarf with dark green leaves ; and r-type plants which resembled A.rosea but exhibited a highly branching habit. The isozyme patterns of the leaves of the pand d-types were similar to those of A.patula, while th.e isozyme patterns of the r-type were either similar to those of A.rosea or different from the parental patterns. Based on anatomical, photosynthetic and enzymological observations, the p- and d-types showed C₃-like traits, whereas the r-type C₄-like ones. Chromosome analysis from the root tips revealed a 2n=36 number in the p- and d-types and a 2n=18 one in the r-type. It is suggested that the expression of C₄ photosynthesis is affected by the dose of the genome of the C₄ species and that the genes related to C₄ photosynthesis tend to be inherited as a group in the interspecific hybrids.