In the present article two aspects dealing with the relations of polyploidy and stcmata characters in Citrus and related genera are dealt with. First the data of Hirano (5) concerning stomata density in numerous forms of the sub-family Aurantioidex are analysed in relation to their chromosomal constitution. It was thought that the- great variability of stomata number per unit area, could possibly be explained by differences in chromosome numbers. Chromosome counts of numerous plants utilized by Hirano at the Citrus Exp. Sta., Riverside, California, were made and all available bibliographical informations concerning the cytological constitution of those species and genera were gathered, whose chromosomes could not be counted at Riverside ; it was concluded that all types studied by Hirano, have 18 somatic chromosomes except for two varieties of Citrus aurantifolia, Swingle, which are triploid. This was the only instance in which differences in chromosome numbers where primarily responsable for the differences in stomata density. In the rest of the material the variability of this character must bo due to differences in genetic constitution. In the succeding chapters the results of a detailed analysis of stomata area and its variability in diploid, triploid and tetraploid Citrus forms are presented; this was done in order to find a simple method of identifying these, polyploids without making aciual chromosome counts. A total of 36 individuals of 5 different species were examined. Listing the average stomata areas, starting with the largest and finishing with the smallest, a continous series is obtained, the diploids, triploids and tetraploids being however associated in three groups, no overlapping from one group to the other having been noticed. Significant differences have been found not only between the general averages of the three groups, but also between the extremes at the limits of two succeeding groups and also in each of them. Conspicuous variability of this character is also noticed in each cf most of the species and its diploid and tetraploid groups. In spite of this variability it is concluded that the determination of stomata area can be useful to geneticists and plant breeders working with Citrus, even when one has to distinguish between triploids and tetraploids in the same progeny, if one knows the stomata area of the related diploids grown under identical circumstances.