Laser flow cytometry was used to analyze nuclear DNA contents (2C values) of five genera (Severinia Ten.,Atalantia Correa, Fortunella Swing., Poncirus Raf., and Citrus L.) taxonomically grouped in subtribe Citrinae (citrus fruit trees) of the Rutaceae, The genotypes analyzed had 2C values ranging from 0.67 pg for diploid Severinia buxifolia (Poir.) Ten. to 1.27 pg for tetraploid Hongkong Fortunella hindsii Swing. There was no significant difference in the 2C values within the sexually compatible diploid species of 11 "true citrus fruit trees" [Citrus aurantium L., C. grandis (L.) Osbeck, C. limon (L.) Burm. f., C. limonia Osbeck, C paradisi Macf,, C. reshni Hort. ex Tanaka, C. sinensis (L.) Osbeck, C. volkameriana Ten. & Pasq., Poncirus trifoliata (L.) Raf., and the intergeneric hybrid C. sinensis x P. trifoliata]. The species Atalantia ceylanica (Arn.) Oliv. (a "near-citrus fruit tree"), sexually incompatible with Citrus spp., had a 2C value significantly different from those of the true citrus fruit tree species. The 2C value of Severinia buxifolia (a "primitive citrus fruit tree"), another species sexually incompatible with the Citrus spp., also differed from those of some of the true citrus fruit tree species. The data largely corresponds with taxonomical differences between a) the genera Citrus and Poncirus and b) the genera Severinia and Atalantia, all assigned to subtribe Citrinae.