The incidence of flavonoids and phenolic acids substantiates the reclassification of a number of diploid taxa formerly considered as varieties of the tetraploid species,L. corniculatus.Whereas leaves ofL. corniculatuscontained leucodelphinidin, quercetin, leucocyanidin, kaempferol,p‐coumaric acid and ferulic acid, those of the diploid species,L. alpinus, L. japonicus, L. kryloviiandL. schoelleri,lacked leucodelphinidin but contained sinapic acid which was rarely found in leaves ofL. corniculatus.The leaves ofL. filicauliscontained the same compounds as those of the aforementioned diploid species but lacked sinapic acid.Lotus tenuisandL. borbasiidiffered most fromL. corniculatusin that their leaves possessed only kaempferol and ferulic acid. The morphological polymorphism evident in plants ofL. corniculatuswas not reflected in the flavonoid and phenolic acid content of this species. The data obtained lend support for the hypothesis of an allotetraploid origin forL. corniculatus.The phenolic content of the leaves of interspecific hybrids between diploid species was a summation of the phenolic residues found in the parental species. The same is true of the plants of an F2population of a cross betweenL. japonicusandL. filicaulis.The results indicate that phenolic compounds in the leaves ofLotusare inherited in a dominant fashion and that they may be used as species characteristics in systematic studies.Lotus caucasicushas been determined to be a tetraploid species with 24 somatic chromosomes.