UDP-N-acetylglucosamine 3-O-acyltransferase (LpxA) catalyzes the transfer of (R)-3-hydroxymyristic acid from its acyl carrier protein thioester to UDP-N-acetylglucosamine. LpxA is the first enzyme in the lipid A biosynthetic pathway and is a target for the design of antibiotics. The x-ray crystal structure of LpxA has been determined to 2.6 angstrom resolution and reveals a domain motif composed of parallel β strands, termed a left-handed parallel β helix (LβH). This unusual fold displays repeated violations of the protein folding constraint requiring right-handed crossover connections between strands of parallel β sheets and may be present in other enzymes that share amino acid sequence homology to the repeated hexapeptide motif of LpxA.