Macroalgae (or seaweeds) are considered potential biomass feedstocks for the production of renewable fuels and chemicals. Their sugar composition is different from that of lignocellulosic biomasses, and in green species, includingUlva lactuca, the major sugars arel-rhamnose andd-glucose.C. beijerinckiiDSM 6423 utilized these sugars in aU. lactucahydrolysate to produce acetic acid, butyric acid, isopropanol, butanol, and ethanol (IBE), and 1,2-propanediol.d-Glucose was almost completely consumed in diluted hydrolysates, whilel-rhamnose ord-xylose was only partially utilized. In this study, the metabolism ofl-rhamnose byC. beijerinckiiDSM 6423 was investigated to improve its utilization from natural resources. Fermentations ond-glucose,l-rhamnose, and a mixture ofd-glucose andl-rhamnose were performed. Onl-rhamnose, the cultures showed low growth and sugar consumption and produced 1,2-propanediol, propionic acid, andn-propanol in addition to acetic and butyric acids, whereas ond-glucose, IBE was the major product. On ad-glucose–l-rhamnose mixture, both sugars were converted simultaneously andl-rhamnose consumption was higher, leading to high levels of 1,2-propanediol (78.4 mM), in addition to 59.4 mM butanol and 31.9 mM isopropanol. Genome and transcriptomics analysis ofd-glucose- andl-rhamnose-grown cells revealed the presence and transcription of genes involved inl-rhamnose utilization and in bacterial microcompartment (BMC) formation. These data provide useful insights into the metabolic pathways involved inl-rhamnose utilization and the effects on the general metabolism (glycolysis, early sporulation, and stress response) induced by growth onl-rhamnose.IMPORTANCEA prerequisite for a successful biobased economy is the efficient conversion of biomass resources into useful products, such as biofuels and bulk and specialty chemicals. In contrast to other industrial microorganisms, natural solvent-producing clostridia utilize a wide range of sugars, including C5, C6, and deoxy-sugars, for production of long-chain alcohols (butanol and 2,3-butanediol), isopropanol, acetone,n-propanol, and organic acids. Butanol production by clostridia from first-generation sugars is already a commercial process, but for the expansion and diversification of the acetone, butanol, and ethanol (ABE)/IBE process to other substrates, more knowledge is needed on the regulation and physiology of fermentation of sugar mixtures. Green macroalgae, produced in aquaculture systems, harvested from the sea or from tides, can be processed into hydrolysates containing mixtures ofd-glucose andl-rhamnose, which can be fermented. The knowledge generated in this study will contribute to the development of more efficient processes for macroalga fermentation and of mixed-sugar fermentation in general.