Microfluidic device detections of E. coli K12 in deionized (DI) water and E. coli in field water sample were demonstrated through static light scattering of latex immunoagglutination using proximity optical fibers. This method is a fully-automated, one-step detection, and requires neither sample pre-treatment nor cell culturing often required in many on-chip detections. We have used highly carboxylated polystyrene submicron latex particles without surfactants to enhance diffusional mixing and prevent non-specific bindings towards successful demonstration of latex immunoagglutination in microfluidic device. Detection of E. coli was performed by taking microscopic images from the view cell of a microfluidic device and counting the fractions of non-agglutinated and agglutinated particles. The limit of detection (LOD) was ca. 150 CFU ml -1 with this method for both E. coli K12 in DI water and E. coli in field water sample, indicating no non-specific bindings. Improved LOD of < 4.3 CFU ml-1 was achieved by measuring forward static light scattering from microfluidic device, using proximity optical fibers and a USB-powered miniature spectrometer. The total assay time for sample preparation (mostly dilutions) and on-chip assay (mostly injections and short incubation time) was < 10 min.