We developed a microsensor for the detection of β-radiation with submillimeter spatial resolution. The microsensor consists of a scintillating sphere (diameter 100-300 μm) glued to a tip of an optical fiber (core diameter 140 μm) enclosed in a thin glass capillary. The scintillating sphere was made by embedding YLu scintillation powder and TiO2 light scattering particles in a methacrylate matrix. Photons generated in the scintillator by the β-particles are guided through the fiber and detected by a photomultiplier connected to the opposite fiber end. The photon counts generated by the photomultiplier are linearly proportional to the volume-specific radioactivity in the sample. The background signal can be suppressed practically to zero if two fibers connected to two photomultipliers working in coincidence are used. This, however, results in slightly diminished sensor performance (lower sensitivity and signal-to-noise ratio). As examples of applications, the diffusion coefficients of 36Cl-, H 14CO3-, and 45Ca2+ in agar-solidified saline water were measured, and the depth distribution of primary productivity in an intact microbial mat was determined from vertical profiles of 14C after incubation in the light using H 14CO3- as tracer. ©; 2007, by the American Society of Limnology and Oceanography, Inc.