C-value was applied as a new criterion to investigate the relationships within the genus Agave L. (Agavaceae). Samples representing nearly 40 taxa were investigated, augmented with data from the literature. Species of Agave have the same basic chromosome number, x = 30, but there are several polyploids. Their somatic nuclear DNA contents (2C), as measured by flow cytometry with propidium iodide falls in a narrow range around 8 pg for the diploids. A comparison is made with nuclear DNA amounts of other related genera. Hosta, with a chromosome complement identical to that of Agave, with 5 large and 25 small chromosomes in the haploid set, is at the base of the Agavaceae clade and has two to three times the amount of nuclear DNA of the other genera.