The mode of inheritance of resistance to bipyridyl herbicides in bipyridyl-resistant biotypes of Arctotheca calendula and of Hordeum leporinum was investigated. F1 plants from reciprocal crosses between diquat-resistant and -susceptible plants of A. calendula showed an intermediate response to diquat application that was nuclearly inherited. Treatment of F2 plants with 100 g ai ha(-1) of diquat or 800 g ai ha(-1) of paraquat killed all homozygous-susceptible plants, caused severe injury to heterozygous plants but only slight or no injury to homozygous-resistant plants. Back crosses of F1 to susceptible plants exhibited intermediate and susceptible phenotypes. The observed segregation ratios in F2 and test-cross populations fitted predicted segregation ratios, 1:2:1 (R:I:S) and 1:1 (I:S) respectively, showing that bipyridyl resistance is conferred by a single incompletely-dominant gene. Biotypes of paraquat-resistant and -susceptible H. leporinum were crossed reciprocally. F1 plants from reciprocal crosses showed an intermediate response to paraquat application. The F2 progeny showed segregation ratios that fitted the predicted segregation ratio of 1:2:1 (R:I:S) forinheritance of resistance being governed by a single partially-dominant gene.