The molecular evolution of two components of elongation factor-1 (EF-1), EF-1β and EF-1δ was analysed using the distance matrix, the maximum parsimony and the maximum likelihood methods, after careful alignment of protein and cDNA sequences. The topology of the phylogenetic trees obtained supports monophyly of plant EF-1β and EF-1β' sequences, and monophyly of higher eukaryotic animal EF-1β and EF-1δ sequences. EF-1β and EF-1δ are homologous in their C-terminal domain. EF-1δ, which emerged before arthropods, originates from a β-type ancestor gene and fusion with a leucine zipper N-terminal motif. Plant EF-1β and EF-1β' correspond to paralogous genes whose ancestor was most likely duplicated before the emergence of monocotyledons and dicotyledons.