Numerous genetic studies have indicated the instability of the high oleic characteristic in sunflower. A high-oleic sunflower mutant was obtained using a dimethyl sulfate chemical treatment. There are several hypotheses about the number and action of genes that control high oleic acid content in sunflower oil. Since most mutations cause changes at the chromosome level, a cytogenetic study (analyses of meiosis and pollen viability) was undertaken to analyze a normal line, an isogenic mutant line and their F1 hybrid. The results of the cytogenetic analyses showed no change in either the number or the structure of chromosomes in the mutant and the F| hybrid in relation to the inbred line. It was concluded that there were no chromosome aberrations such as heterozygous translocation or inversion. Histological analyses showed no difference in the content of the spongy and palisade parenchyma tissues of the cotyledons in the normal line, the mutant and their F1 hybrid.