Viable cell count was used to determine whether Metschnikowia sp. C14 can colonize the intestine of juvenile sea cucumber Apostichopus japonicus. Sea cucumber individuals were divided into two groups, which were fed the control diet for 38 days or the C14-supplemented diet at 10 5 cells g −1 diet for 28 days, then the control diet from day 29 to day 38. The number of C14 cells in the intestine of sea cucumber fed the C14-supplemented diet significantly increased from day 7 to day 28, and decreased from day 29 to day 38. Sea cucumber fed with the diet containing C14 showed a significant increase in trypsin activity and lipase activity from day 21 to day 33 compared with the control. Feeding C14 significantly improved the phagocytic activity and respiratory burst in coelomocytes from day 21 to day 35 and from day 14 to day 38, respectively. In addition, there was an obvious enhancement in lysozyme activity (from day 21 to day 38 or day 33), phenoloxidase activity (from day 21 to day 28) and total nitric oxide synthase activity (from day 14 to day 38) in coelomic fluid supernatant and/or coelomocyte cell lysate supernatant compared with the control. There were significant positive correlations between the number of C14 cells colonizing the intestine and trypsin activity of the intestine, lysozyme activity of the coelomic fluid supernatant and coelomocyte lysate supernatant from sea cucumber. These data suggested that the number of C14 cells should be maintained at 10 5 cfu (colony-forming units) g −1 intestine material for the maximum benefit.