Using a degenerate primer designed from triterpene synthase sequences, we have isolated a new gene from the medicinal plant Artemisia annua. The predicted protein is highly similar to beta-amyrin synthases (EC 5.4.99.-), sharing amino acid sequence identities of up to 86%. Expression of the gene, designated AaBAS, in Saccharomyces cerevisiae, followed by GC/MS analysis, confirmed the encoded enzyme as a beta-amyrin synthase. Through engineering the sterol pathway in S. cerevisiae, we explore strategies for increasing triterpene production, using AaBAS as a test case. By manipulation of two key enzymes in the pathway, 3-hydroxy-3-methylglutaryl-CoA reductase and lanosterol synthase, we have improved beta-amyrin production by 50%, achieving levels of 6 mg.L(-1) culture. As we have observed a 12-fold increase in squalene levels, it appears that this strain has the capacity for even higher beta-amyrin production. Options for further engineering efforts are explored.