Abstract: The present invention provides a method for producing. a-hydroxy acid ammonium salt that uses a specific microbial strain capable of accumulating ?-hydroxy acid ammonium salt at a high concentration while also maintaining an industrially satisfactory production rate for a long period of time. More specifically, the present invention uses a microbial catalyst originating in a microbial strain which, during conversion of ?-hydroxynitrile to ?-hydroxy acid ammonium salt, is capable of maintaining the average production rate of ?-hydroxy acid ammonium salt at at least 100 ?mol/min per g of dry microbial cell weight for 14 days or more without adding fresh microbial catalyst, and is capable of accumulating the ?-hydroxy acid ammonium salt at 20 to 60% by weight. An example of this microbial strain is Arthrobacter sp. strain NSSC204.