l-(−)-Carnitine is a ubiquitously occurring substance, essential for the transport of long-chain fatty acids through the inner mitochondrial membrane. Bacteria are able to metabolize this trimethylammonium compound in three different ways. Some, especially Pseudomonas species, assimilate l-(−)-carnitine as sole source of carbon and nitrogen. The first catabolic step is catalysed by the l-(−)-carnitine dehydrogenase. Others, for instance, Acinetobacter species, degrade only the carbon backbone, with formation of trimethylamine. Finally, various members of the Enterobacteriaceae are able to convert carnitine, via crotonobetaine, to γ-butyrobetaine in the presence of C and N sources and under anaerobic conditions. This two-step pathway, including a l-(−)-carnitine dehydratase and the crotonobetaine reductase, was demonstrated in Escherichia coli. The DNA sequence encompassing the cai genes of E. coli, which encode the carnitine pathway, has been determined. Some bacteria are also able to metabolize the non-physiological d-(+)-carnitine, which results as a waste product in some chemical procedures for l-(−)-carnitine production based on the resolution of racemic carnitine.