Estudos citológicos no gênero ArachisAll the varieties of peanuts previously studied have been found to have 2n=40 chromosomes and belong to the species Arachis hypogsea L. and A. nambiquarse Hoehne. In the present paper the author confirms the chromosome number previously reported and reveals the existence of other species of -peanuts having 2n = 20 chromosomes. The species having 2n = 20 chromosomes were found to be: A. prostrata Benth (?), A. marginata Gardn., A. marginata Gardn. var. submarginata Hoehne, A. Diogoi Hoehne f. typica Hoehne, .A. Diogoi Hoehne sub-esp. major Hoehne and A. villosulicarpa Hoehne. In eight different samples of peanuts seeds sent from the State of Mato Grosso, seven were found to have 2n =40 chromosomes and one sample had 2n = 20 chromosomes. Unfortunately it has not yet been possible to obtain the botanical classification of these peanut samples. A chromosomal chimaera observed in a root tip section of A. hypogxa showed sectors having 40 and 80 chromosomes. Based upon the genet ical studies made by several authors, and under the light of the present investigation, the cultivated peanut must be considered of a tetraploid nature. It seems likely that A. hypogxa probably arose spontaneously by chromosome doubling from diploid forms. It also seems probable that this took place in the State of Mato Grosso, Brasil, which is generally recognised as place where peanut plants are indigenous.