Abstract: This work is aimed to investigate the morphology and molecular biology of the endophytic microbacilli in the intestine of Periplaneta americana，to study its antibacterial activity，and to analyze its potential of producing halogenated secondary metabolites at the gene level. The endophytic microbacillus in the intestine of P. americana was isolated and purified by plate scribing. After Gram staining，the cells were observed by microscopy. The 16S rDNA PCR，BLAST homology alignment and phylogenetic tree were used to identify the isolated strains in molecular biology and to screen key enzyme genes for antibiotics synthesis. Eight strains of common pathogenic bacteria were used as indicator bacteria，and the antibacterial activities were primarily tested by the method of Oxford cup. The colony colors of 8 isolated endophytic microbacilli were light yellow or white. The results of 16S rDNA PCR and BLAST homology alignment showed that the eight microbacilli had high homology with known microbacilli at 98%；while the results of phylogenetic tree demonstrated that they had high homology with the microbacilli from soil and marine. Screening of key enzyme genes in antibiotic synthesis revealed that 6 strains of above 8 presented expressions to 2 of FADH2-dependent halogenase，NRPS，and PKSI. Bacteriostatic experiments showed that 6 strains of 8 presented selective inhibitions to the growth of tested fungi and bacteria at varied level，indicating that there exist endophytic microbacilli in the intestine of P. americana.

Key words: Periplaneta americana, endophytic microbacillus, 16S rDNA phylogenetic tree, antibacterial activity