To investigate the origin of cultivated trumpet daffodils the genome size (2C-value) of more than 100 old and new cultivars were measured. A large number of concolored yellow tetraploid cultivars with large coronas seem to be derived from a doubling of chromosomes of a hybrid of the two species N. hispanicus Gouan and N. pseudonarcissus L. This can also still be recognized by the presence of a black spot at the top of the anthers in about 15 % of the tetraploid cultivars. Assuming N. pseudonarcissus as one of the parents of the allotetraploid cultivars, species of trumpet daffodils of section Pseudonarcissi investigated earlier were compared. Doubling of the nuclear DNA content of 23.8 pg of N. pseudonarcissus falls short of the values found for the trumpet daffodils. Species of trumpet daffodils of section Pseudonarcissi with a complementing higher amount of nuclear DNA like N. hispanicus with 25.8 pg, N. poeticus L. with 26 pg and the hexaploid N. pseudonarcissus ssp. bicolor (L.) Baker with 67.7 pg (Zonneveld,2008) and other species were evaluated. N. hispanicus with 25.8 pg clearly differs from N. pseudonarcissus with 23.8 pg and is accepted here also as a species. The nuclear DNA content of N. bujei (Fern. Casas) Fern. Casas with 30 pg fits with the (ancient) hybrid origin for N. bujei between N. longispathus Pugsley (36 pg) and N. hispanicus Gouan (25.8 pg). The tetraploids with white tepals and yellow corona’s can be obtained by crossing a diploid N. pseudonarcissus with the hexaploid and bicolored N. pseudonarcissus ssp. bicolor (L.) Baker.