The nucleotide sequence and predicted amino acid sequence of malate dehydrogenase (decarboxylating) or malic enzyme (EC 1.1.1.40) of the amitochondriate protist Giardia lamblia were determined. The overall amino acid identity with malic enzyme sequences from other eukaryotes was between 34 and 39%. Functional domains previously defined in other malic enzymes, the malate-, the ADP- and the NAD(P)-binding domains, were present also in the G. lamblia sequence. In phylogenetic reconstructions, the G. lamblia sequence is part of the eukaryotic clade, but its relative position versus the other early branches of the eukaryotic tree (Trichomonas vaginalis hydrogenosome and plant mitochondria) cannot be firmly established. The results indicate, however, a long, independent evolutionary past of this enzyme.