Provided herein are compositions and methods for producing myrcene by culturing genetically modified microbial host cells that express a myrcene synthase and optionally a geranyl pyroplosphate synthase. Also provided herein are isolated nucleic acid molecules that encode myrcene synthase variants derived from the Ocimum species myrcene synthase, which comprise one or more amino acid substitutions that improve in vivo performance of myrcene synthase in genetically modified microbial host cells. Also provided herein are isolated myrcene synthase variants that exhibit an improved activity for converting geranyl diphosphate into myrcene.