INTRODUCTION The growing prevalence of antibiotic resistance due to its extreme use in poultry production is an emerging public health issue. This results into severe restriction or total ban on the use of antibiotics in animal and poultry industry in many countries. Consequently, an urgent need is felt globally to look for alternatives to antibiotics for maintaining health and performance under commercial conditions. Probiotics, a microbial culture broth containing antimicrobial compounds, are gaining attention as alternative to antibiotics because of their safety and potentials for improving animal health (Fuller, 1989). Several microorganisms, under the name of "probiotics", have been proposed and used in a wide range of field trials, with the objective to improve growth performance (Khaksefidi and Ghoorchi, 2006; Mountzouris et al., 2007; Awad et al., 2009; Shim et al., 2010), nutrient retention (Li et al., 2008; Mountzouris et al., 2010), caecal microbial balance (Teo and Tan, 2007; Mountzouris et al., 2010) and intestinal morphology (Awad et al., 2009, 2010). Due to the numerous applications of probiotics, attempts are being made to screen and identify new probiotic strains with particular characteristics that would be appropriate for specific applications. Among several bacterial species used as probiotics, spore forming Bacillus spp. has been preferred due to high resistance of its spores to harsh environment and long term storage at ambient temperature (Fuller, 1989; Hong et al., 2005). However, there is very limited information on the effects of Bacillus subtilis on nutrient retention, caecal microbial population and intestinal morphology in broilers.