Summary:Inheritance of resistance to simazine was investigated in reciprocal pair crosses of resistant and susceptible genotypes ofSenecio vulgaris. The simazine resistant genotypes had been selected in a screening experiment and were unaffected by 2.8 kg ha−1˜’a.i. simazine. Reciprocal differences for resistance and susceptibility to simazine inF2families showed that resistance is maternally inherited. The practical implication of maternal inheritance is that a resistant field population will build up very rapidly from a very low initial frequency of the resistant geno