Resistance to the non-selective herbicide, glyphosate, has evolved recently in several populations of Lolium rigidum (Gaud.). Based upon the observed pattern of inheritance, glyphosate resistant and susceptible populations are most probably homozygous for glyphosate resistance and susceptibility respectively. When these populations were crossed and the F-1 progeny treated with glyphosate, the dose response behavior was intermediate to that of the parental populations. This observation, coupled with an absence of a difference between reciprocal F-1 populations, suggests that glyphosate resistance is inherited as an incompletely dominant nuclear-encoded trait. The segregation of resistance in F-1 x S backcrosses suggests that the major part of the observed resistance is conferred by a single gene, although at low glyphosate treatments other genes may also contribute to plant survival. It appears from this study that a single nuclear gene confers resistance to glyphosate in one population of L. rigidum.