<jats:title>Abstract</jats:title><jats:p>In <jats:italic>Escherichia coli</jats:italic> the osmoprotective compound glycine betaine is produced from choline by two enzymes; choline dehydrogenase (CDH) oxidizes choline to betaine aldehyde and then further on to glycine betaine, while betaine aldehyde dehydrogenase (BADH) facilitates the conversion of betaine aldehyde to glycine betaine. To evaluate the importance of BADH, a BADH/CDH fusion enzyme was constructed and expressed in <jats:italic>E. coli</jats:italic> and in <jats:italic>Nicotiana tabacum</jats:italic>. The fusion enzyme displayed both enzyme activities, and a coupled reaction could be measured. The enzyme was characterized regarding molecular weight and the dependence of the enzyme activities on environmental factors (salt, pH, and poly(ethylene glycol) addition). At high choline concentrations, <jats:italic>E. coli</jats:italic> cells expressing BADH/CDH were able to grow to higher final densities and to accumulate more glycine betaine than cells expressing CDH only. The intracellular glycine betaine levels were almost 5‐fold higher for BADH/CDH when product concentration was related to CDH activity. Also, after culturing the cells at high NaCl concentrations, more glycine betaine was accumulated. On medium containing 20 mM choline, transgenic tobacco plants expressing BADH/CDH grew considerably faster than vector‐transformed control plants.