Pseudomonas sp. LS13-1 was isolated as a producer of lactobionic acid from whey and when grown with 207 g whey l(-1) (150 g lactose l(-1) equivalent) and three intermittent additions of 69 g whey l(-1) (50 g lactose l(-1) equivalent) in a fed-batch culture at pH 5.5 in a 2-l jar fermenter, it produced 175 g lactobionic acid l(-1) after 180 h. In a lactose medium it produced 240 lactobionic acid l(-1) from a total of 300 g lactose l(-1) after 155 h. With the addition of 20 CaCO3 l(-1) instead of pH control, 290 g lactobionic acid l(-1) was produced in the lactose medium after 155 h with a yield of higher than 90% (mon mol(-1)).