['To investigate irregular colony morphology formation in Salmonella enterica serovar Typhimurium DPC6046 in the presence of a lytic phage, Felix 01.', 'Phage-resistant derivatives of the parent strain DPC6046 were isolated which exhibited an irregular colony morphology. These were subjected to viability studies by using confocal scanning laser microscopy and live/dead BacLight stain to evaluate the cell viability within the colony. The phenomenon was also observed with other S. enterica serotypes tested which were normally sensitive to phage Felix. In the case of strain DPC6046, dead cells were clearly evident at the irregular edges of the phage-resistant colonies in locations where the cell density was lower. This colony morphology was not apparent with two other Salmonella phages tested.'] Do salmonella enterica phage-resistant mutant colonies display an unusual phenotype in the presence of phage Felix 01?