l‐Serine dehydratase (SD) was produced to the extent of only 0·04 international unit (iu) per mg dry weight byAeromonos punctataNRRL B‐928 when grown on a chemically defined medium. Addition of 2% (w/v)l‐serine to this medium increased SD ten‐fold or more, indicating a significant inductive effect.d‐Serine was toxic to the organism, making weights and enzyme titres uncertain, butdl‐serine induced a specific activity of SD at least equal to that from thelenantiomorph. With complex media containing beef extract, the toxic effect ofd‐serine was partially overcome, and sufficient growth was obtained to permit reliable analytical figures;d‐serine induced slightly better specific activities than thelisomer.dl‐Serine at 2% concentration induced a specific activity of 1·2 iu/mg, twice that ofl