The Giardia lamblia triosephosphate isomerase (TIM) gene was cloned using a probe generated by a polymerase chain reaction that employed primers complementary to highly conserved regions of TIM. The nucleotide sequence predicts a protein that is 38 and 47% identical to TIM from prokaryotic and eukaryotic sources, respectively. Like all other Giardia protein-coding genes studied thus far, the TIM gene lacks introns and is transcribed to yield a polyadenylated mRNA with an extremely short 5' untranslated region. The Giardia TIM gene complemented an Escherichia coli triosephosphate isomerase deletion mutant. The simplicity and success of complementation suggests its general utility in cloning Giardia genes of known function.