The invention provides a modified tubulin comprising a tubulin polypeptide which confers resistance to an anti-tubulin agent when expressed within a biological organism, said polypeptide having at least one amino acid substitution such that hydrophobicity is increased at one or more positions in the polypeptide. DNA sequences encoding such modified tubulins may be used to confer resistance on organisms which otherwise would be susceptible to one or more anti-tubulin agents. For example, a plant which is resistant to a herbicide that disrupts microtubule structure or function may be produced.