Black tiger shrimp, Penaeus monldon, cultivated in 2 field trials in a 2.25 m2 net cages located in 1,000 m2 earthen pond, were fed with commercial feed supplemented with B. subtilis P11 in order to find out its influence on shrimp growth and survival. In both trials, growth rate of probiotic shrimp were significantly higher (P<0.05) than those of the control groups, but the survival were not significant between the two groups. Challenge tests on shrimp after being tainted with Vibrio harveyi strain 639 revealed that cumulative mortality of the probiotic feeds shrimps cultivated after 120 days (first cultivation) were significantly lower (P<0.05) than those of the control group while those cultivated after 80 days (secondary cultivation) were non significant (P<0.05) as compare to those of the control group. However, the mortality rate of probiotic treatment of the 2nd trial was slower. Immunity testing of total hemocyte and antibacterial activity on shrimp before and after 2 days of challenge test was conducted. Interestingly, the total hemocyte count and antibacterial activity of the probiotic shrimps before the challenge test were higher than those of the control. Decrease in total hemocyte from ~107 cell ml-1 to ~106 cell ml-1 and increase in antibacterial activity after challenge tests among shrimps in two treatments were observed.