1. The chromosome number of fourteen species (twenty forms) belonging to five different genera of the subfamily Aurantioidex. are reported. All determinations available in the literature on chromosome numbers made in this subfamily are tabulated. The basic chromosome number is x = 9 for all forms so far investigated. About 40 triploids, some 190 tetraploids (including 19 in Poncirus, 1 in Triphasia, and 1 wild form in Fortunella), 1 pentaploid, 1 hexaploid, and 14 or 15 aneuploids have been reported. 2. All these polyploids, except the 21 tetraploids just mentioned and several triploid hybrids between Citrus and Fortunella, belong to the genus Citrus. Considering the genetic similarity of the chromosome complements of several genera of the subtribe Citrinx, it is suggested that some of these might perhaps better be considered mere species of a single genus.