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Genome size was estimated using Feulgen densitometry for 76 accessions of 40 taxa of Orobanche and two taxa each of the related genera Phelypaea and Cistanche, providing the first data set for any group of nonphotosynthetic angiosperms. The 2C‐values were 16.8–19.9 pg in Cistanche, 2.9–11.6 pg in Orobanche sect. Orobanche, 6.8–10.8 pg in sect. Trionychon, 4.3–5.1 pg in sect. Myzorrhiza, and 4.9–5.8 pg and 10.5 pg in the two diploid species Phelypaea coccinea and P. tournefortii, respectively. Distribution of genome size is congruent with phylogenetic lineages identified by analyses of nuclear ITS sequence data, in particular regarding the distinctness of O. anatolica from the rest of sect. Orobanche. With the exception of tetraploid O. transcaucasica, polyploid taxa of sect. Orobanche are among those with the smallest C‐values, suggesting substantial genome downsizing after polyploidization (as in other …