Phytic acid (myo-inositol 1,2,3,4,5,6 hexakisphosphate) is the most abundant form of cereal grain phosphorus (P) and is virtually indigestible by humans or non-ruminant livestock. Induced, recessive mutations that decrease grain phytic acid (PA) content have been isolated and genetically mapped in barley, maize, and rice. The most common type of lpa mutation, designated lpa1, exhibits large reductions of PA and corresponding increases of inorganic phosphorus (Pi). A second type of lpa mutation, designated lpa2, also exhibits large reductions of PA and increased Pi, however a significant amount of P remains bound as lower inositol polyphosphate(s) (mainly myo-inositol pentakisphosphate). Based on the recessive biochemical phenotypes of lpa1 mutants, it has been hypothesized that this type of mutation interrupts inositol synthesis or supply. A maize lpa1 mutation and myo-inositol 1-phosphate synthase (MIPS) gene have been mapped to loci that apparently coincide on maize chromosome 1S, although direct co-segregation experiments are not yet complete. Single copy MIPS genes have also been mapped to loci on barley chromosome 4H and rice chromosome 3, which are evidently orthologous to the MIPS gene on maize chromosome 1S based on comparative mapping studies between these species. However, the lpa1 mutations of these small grain species have been mapped to barley chromosome 4H and rice chromosome 2, and do not appear to be orthologous. The barley lpa2 mutation maps to a locus on barley chromosome 7H and may be orthologous to the maize lpa2 mutation located on maize chromosome 1. In summary these studies define separate genetic map locations for the lpa1, lpa2, and MIPS genes of barley, maize, and rice with one exception, the coincidence of maize lpa1 and MIPS loci on maize chromosome 1S. These studies also suggest lpa2 mutations may be orthologous between barley and maize, whereas lpa1 mutations probably affect at least two different (non-orthologous) genes among barley, maize, and rice.