Inter-simple sequence repeats (ISSR), simple sequence repeats (SSR) and isozymes were used to measure genetic diversity and phylogenetic relationships among 95 Citrus L. accessions including 57 lemons [C. limon (L.) Burm. f.], related taxa, and three proposed ancestral species, C. maxima (Burm.) Merrill (pummelo), C. medica L. (citron), and C. reticulata Blanco (mandarin). The ancestry of lemons and several other suspected hybrids was also studied. Five isozyme and five SSR loci revealed relatively little variation among most lemons, but a high level of variation among the relatively distant Citrus taxa. Eight ISSR primers amplified a total of 103 polymorphic fragments among the 83 accessions. Similarity matrices were calculated and phylogenetic trees derived using unweighted pair-group method, arithmetic average cluster analysis. All lemons, rough lemons, and sweet lemons, as well as some other suspected hybrids, clustered with citrons. Most lemons (68%) had nearly identical marker phenotypes, suggesting they originated from a single clonal parent via a series of mutations. Citrons contributed the largest part of the lemon genome and a major part of the genomes of rough lemons, sweet lemons, and sweet limes. Bands that characterize C. reticulata and C. maxima were detected in lemons, suggesting that these taxa also contributed to the pedigree of lemon.