Intensive screening of a small population of mutagenised wheat lines revealed a large number of lines with altered resistance to both yellow and brown rust. The parental cultivar Guardian has an intermediate level of adult plant resistance to this disease; mutants were therefore isolated that showed either enhanced resistance or enhanced susceptibility to yellow rust. Seven lines were identified that gave an altered yellow rust disease phenotype as adult plants under both field and greenhouse conditions. Simultaneous field testing for brown rust infection identified two of these lines as having increased resistance to brown rust.Key words: Disease resistance, mutants, wheat, yellow rust.