Genome size (C-value) is used as a new criterion to investigate the taxonomy of the genus Aloe L. Nearly all species of the genus Aloe have the same basic chromosome number (2n = 2x = 14). However, the nuclear DNA content, measured in 83 species by flow cytometry with propidium iodide, is shown to range from about 16 to 44 pg in the diploids. This implies that the largest genome contains roughly 3 x 1010 more base pairs than the smallest. Therefore, nuclear DNA content is a very relevant taxonomic trait that can be measured simply by flow cytometry. It is shown that A. plicatilis (L.) Mill. and A. haemanthifolia A.Berger & Marloth have by far the lowest DNA content of all measured Aloe species and might belong to the same section. A. pillansii L. Guthrie has a DNA content similar to A. dichotoma Masson and the following new combinations are made: Aloe dichotoma Masson subsp. pillansii (L. Guthrie) Zonneveld comb. nov. and Aloe dichotoma subsp. ramosissima (Pillans) Zonneveld comb. nov. Aloe distans Haw. and A. comptonii Reynolds clearly belong to A. mitriformis Mill. and are combined here as: Aloe mitriformis Mill. subsp. distans (Haw.) Zonneveld comb. nov. and Aloe mitriformis Mill. subsp. comptonii (Reynolds) Zonneveld comb. nov.