Two component signal transduction systems are ubiquitous in prokaryotic organisms to sense environmental changes and adapt to fluctuating conditions. The two components typically consist of a sensor kinase and a response regulator. The sensor kinase receives a signal from the environment and controls the phosphorylation level of the response regulator. Upon phosphorylation, the response regulator induces a change in the organism to respond to the external stimuli. The regulatory system that controls porin expression in Escherichia coli, for example, is a two component system consisting of the sensor kinase, EnvZ (envelope protein), and the response regulator, OmpR (outer membrane protein regulator). In the past, EnvZ / OmpR interactions have been primarily characterized using its isolated cytoplasmic domain termed EnvZc. The characteristics of EnvZc are thought to mimic EnvZ; however, it is not the full length protein in a membrane environment. Using lysed spheroplasts, we characterized the interaction of full length EnvZ and OmpR (both phosphorylated, OmpR~P and non-phosphorylated, OmpR) in an in vivo system using FRET (fluorescence resonance energy transfer). The binding affinities calculated for OmpR and OmpR~P were 500nM and 1.5uM respectively. The Hill coefficient for OmpR and OmpR~P was 1.2 in either case, suggesting a slightly cooperative interaction. This is a novel result since prior work using EnvZc suggested that the phosphatase activity of EnvZ was negligible.