Basil, Ocimum L., is an economically important member of the family Lamiaceae. It has been widely used for many purposes, such as culinary herbs. The genus harbors large variation in chromosome number and size, which is believed to be a consequence of centuries of cultivation and selection for desirable traits. Meiosis was examined in five Ocimum species from Thailand, from five populations of each species. The results revealed that three of these species are cytogenetically related, forming a polyploid series with a base chromosome number of x=13: O. americanum L. (2n=2x=26, a new number for Thai plants), O. basilicum L. (2n=4x=52) and O. africanum Lour. (2n=6x=78, a new number for the genus). Two species with small chromosomes, O. tenuiflorum L. (2n=36) and O. gratissimum L. (2n=40), seem probably tetraploids with base numbers x=9 and 10, respectively. Meiotic irregularities in O. basilicum and O. africanum suggested that the species may be newly formed polyploids undergoing diploidization. Unlike recent polyploids, the meiotic chromosomes of O. tenuiflorum and O. gratissimum are entirely stable. Statistically significant intraspecific variation in chiasma frequencies was found in O. basilicum and O. africanum and the variation appeared to be geographically associated. The results presented here are useful both in the conservation of native basil species and in the breeding of elite accessions.