Betaine and Na+ symport has been extensively studied in the osmotically regulated transporter BetP from Corynebacterium glutamicum, a member of the betaine/choline/carnitine transporter family, which shares the conserved LeuT-like fold of two inverted structural repeats(1). BetP adjusts its transport activity by sensing the cytoplasmic K+ concentration as a measure for hyperosmotic stress via the osmosensing carboxy-terminal domain(2,3). BetP needs to be in a trimeric state for communication between individual protomers through several intratrimeric interaction sites(4). Recently, crystal structures of inward-facing BetP trimers have contributed to our understanding of activity regulation on a molecular level(5,6). Here we report new crystal structures, which reveal two conformationally asymmetric BetP trimers(7), capturing among them three distinct transport states. We observe a total of four new conformations at once: an outward-open apo and an outward-occluded apo state, and two closed transition states-one in complex with betaine and one substrate-free. On the basis of these new structures, we identified local and global conformational changes in BetP that underlie the molecular transport mechanism, which partially resemble structural changes observed in other sodium-coupled LeuT-like fold transporters, but show differences we attribute to the osmolytic nature of betaine, the exclusive substrate specificity and the regulatory properties of BetP.