The infection of three spring barley cultivars by Puccinia hordei was studied histologically. In cv. Cebada Capa, resistance governed by gene Pa 7 prevents sporulation by the fungus and elicits a visible necrotic host response. Up to that point, penetration and colonization proceed in a way similar to that in cv. Vada, which, compared with the highly susceptible cv. Gold, allows the relatively slow development of fewer and smaller pustules. It is concluded that the slow colonization in Cebada Capa is governed by a similar resistance mechanism to that in Vada. The value of this procedure in the detection of such cryptic resistance, which is masked by Pa gene-governed resistance, is discussed in relation to breeding cultivars with stable, effective resistance to cereal rusts.