We describe imaging of green fluorescent protein (GFP)-expressing bacteria from outside intact infected animals. This simple, non-intrusive technique can show in great detail the spatial-temporal behavior of the infectious process. The bacteria, expressing the GFP, are sufficiently bright as to be clearly visible from outside the infected animal and recorded with simple equipment. Introduced bacteria can be whole-body imaged in most mouse organs, including the peritoneal cavity, stomach, small intestine, and colon. This imaging technology affords a powerful approach to visualizing the infection process, determining the tissue specificity of infection, the spatial migration of the infectious agents and the response to antimicrobial agents.