Tectiviridae is a relative rare group of non-enveloped tail-less phages with a double-layer capsid where the ~15 kb linear dsDNA is located within a lipid-containing membrane/vesicle covered by a rigid icosahedral protein capsid. This family contains two groups of phages: the lytic ones infecting Gram-negative enterobacteria, and the temperate ones preying on Gram-positive bacteria belonging to the Bacillus cereus group. Phages GIL01/Bam35, GIL16, AP50 and Wip1 are the fully-sequenced representatives of the second group. Since, GIL01 and relatives are capable to reside as temperate phages remaining as autonomous linear plasmids in the cell upon infection, it is important to understand the selective pressures undergone by the bacteria when facing this type of phages. This work aims to depict bacterial resistance triggered by the presence of tectiviruses. For this purpose, Bacillus thuringiensis sv. israelensis strain GBJ002 was subjected to a selective pressure after repetitive propagation with clear plaque (CP) mutants of GIL01 and GIL16. The CP mutants showed an elevated efficiency of killing mainly because they propagate exclusively lytically. Twenty tectivirus-resistant bacterial mutants were isolated. These resistant bacteria showed differences in cell and colony morphotypes and displayed distinct adaptation features (i.e. biofilm formation, sporulation rate, swarming motility, differences in metabolic profiles and antibiotic susceptibilities). These observations indicated that tectiviruses may drive life-trait changes and ecological adaptations in B. thuringiensis, as result of a phage-selective pressure. To unravel the genetic changes responsible for tectivirus-resistance in B. thuringiensis, a whole genome sequencing method was approached. Using a pooled high-throughput sequencing analysis of multiple independent bacterial mutants, potential genes causing the resistant phenotype were identified. Several genes associated with cell-wall metabolism and turn-over, as well as cell-surface proteins, have been pinpointed. Currently these candidate genes are being studied intensely to confirm their enrolment in the bacteriophage-resistant phenotype.