Hybrids of ‘Eureka’ and ‘Lisbon’ lemons were studied along with their parents and possible ancestral types to investigate the phylogeny of Citrus limon (L.) Burm. f. Data on leaf, flower, and fruit characters were collected and analyzed for inheritance of specific characters and for the amount of variability. Leaf and rind-oil components were analyzed by gas chromatography. Those hybrids producing sufficient microspores were studied for meiotic abnormalities. Segregation of all characters studied demonstrates the heterozygosity of C. limon. Color (anthocyanin) development in flower buds and young leaves is most likely controlled by a single dominant gene. Color on the young stems may be controlled by two dominant genes, one of which also controls color in flower buds and young leaves. Preformed root initials are most likely controlled by one, and leaf articulation by at least two genes.

Characters specific to C. medica were present in the lemon progeny, indicating definite involvement of the citron in the lemon ancestry. Some characters indicate a possible common gene source with C. aurantifolia and C. jambhiri. The deviation of some hybrids from their parents or proposed ancestors indicates a contributing gene source that has not yet been identified.

The presence of much sterility and wide segregation of characters argues against the specific standing of C. limon. It has a common gene source with the citron-lime group, and it may be a derived hybrid involving some unidentified taxa.

The overlapping geographical distribution of various Citrus taxa at the assumed place of origin of C. limon further suggests the possibility of intercrossing or common origin of various Citrus species.