Abstract The size distribution of starch granules varies between species, with cereal seeds ( Triticaceae) containing either one or two distinct populations of granules. The genetic and biochemical basis for differences in granule dimensions remains unknown, although single-locus mutations conferring high- or low-amylose phenotypes affect granule size. The recessive shx shrunken endosperm mutation of barley is novel, in that the soluble starch synthase activity is drastically lowered, reducing starch content but not greatly altering amylose content. To help understand the relationship between starch synthetic activity and granule biosynthesis, granule size, number and properties were analysed for normal and shx barley nearly isogenic in a cv. Bomi background. Starch in mature shx displayed an altered granule size distribution, consisting of B-granules and smaller A-granules. Developing and mature seeds of shx barley contained fewer granules than normal seeds of the same age. The amylose content of shx was somewhat higher than normal in developing seeds, remaining 2% higher at maturity.