Ridolfia is a monotypic genus native to the Mediterranean area and Macaronesia. Its single species, Ridolfia segetum, is rather weedy and often occurs as an alien beyond its native distribution range.
Molecular studies confirmed the close relationship of Ridolfia and the cultivated species of Ammi, Anethum, Apium, Foeniculum, and Petroselinum (Downie & al. 2000).
Downie S.R., Katz-Downie D.S. & Watson M.F. (2000) A phylogeny of the flowering plant family Apiaceae based on chloroplast DNA rpl16 and rpoC1 intron sequences: towards a suprageneric classification of subfamily Apioideae. Amer. J. Bot. 87(2): 273-292. [available online at: http://www.amjbot.org/content/87/2/273.long]