The concept of using probiotics derived from humans in aquaculture for growth promotion and infectious control has been received with great interest. However, a major concern of using probiotics is their viability during transit through the diverse conditions of the aquatic animal's gut. This study describes the application of microencapsulation technique on a human-derived probiotic, Lactobacillus rhamnosus GG (LGG), and to determine the cell viability and morphology of the microcapsules while in transit through the tilapia gastrointestinal tract. The effect of feeding the microencapsulated probiotic on tilapia growth performance, intestinal morphology and survival against streptococcal infection was also evaluated. The in vitro results showed that the cell viability of both the alginate matrix and skim milk alginate matrix microencapsulated probiotics was maintained after storage at room temperature for 14 days, while viable free LGG could not be detected after only 7 days. The viability of microencapsulated probiotics (alginate matrix and skim milk alginate matrix) under simulated gastric conditions and in tilapia bile was significantly higher than that of the free probiotic. The morphological evidence by scanning electron microscopy revealed the protective effect of alginate capsule under tilapia gastrointestinal condition. The results from the in vivo study revealed that the microencapsulated probiotic improved overall intestinal structure, growth performance and significant protection against Streptococcus agalactiae challenge. These results suggest the potential application of alginate-microencapsulated LGG in tilapias. (C) 2015 Published by Elsevier B.V.