An eight-week trial was performed to evaluate three commercial/prototype probiotics supplemented in red hybrid tilapia,Oreochromissp. diets. Triplicate groups of tilapia were measured for growth performance, feeding efficiencies, and whole-body composition. After the feeding trial, duplicate groups of tilapia were assessed for their resistance toStreptococcus agalactiaeover 23 days. Six diets were supplemented with 0.1% or 0.3% PB1consisting ofBacillus subtilis, 0.1% or 0.3% PB2consisting ofB. licheniformisor 0.1% MPB consisting ofBacillussp. andPediococcussp. Probiotics had no effect (p > 0.05) on growth or feeding efficiencies, although whole-body crude protein was significantly higher in the PB10.3% treatment. Tilapia in the probiotic treatments had a higher resistance toS. agalactiaeand, with the exception of the PB20.1% diet, were all significantly higher than the control treatment. Although the tested probiotics were not growth promoters, dietaryB. subtiliswas the most effective prophylactic against pathogenic bacteria. [ABSTRACT FROM AUTHOR]