One well-developed paradigm for biosensing uses living microorganisms as sensors for environmental stimuli. In this paradigm, engineered cells contain plasmid or chromosomal sequences that link stress-inducible promoters to the production of fluorescent reporter proteins. We are developing such a sensor system with an aim for portability. This work describes the development and performance of a compact optical system designed for low-power detection of fluorescent reporter proteins in microliter-scale cell suspensions. Light-emitting diodes (LEDs) and silicon photodetectors (PDs) were configured to measure red fluorescence, green fluorescence, and cell biomass pseudosimultaneously in 100 μl samples. The optical detectors were calibrated using E. coli cells that expressed red and green fluorescent proteins (dsRed2 and gfp-asv, Clontech) either constitutively or through chemical induction. We show that sufficient sensitivity for certain whole-cell biosensor applications is achievable in low-volume samples, despite the simplicity and low-cost nature of the detector system. The prototype optical detector occupies approximately 1.8 cm x 2.2 cm x 3.0 cm.