A microscope‐based fluorescence correlation apparatus for diffusivity measurements is described. The use of optical fibers offers not only the advantage of setup flexibility but also an improved quality of the illuminating beam. The achieved beam radius is as small as 0.4 &mgr;m, and the size of the observed volume amounts to only 10 &mgr;m3. The measured correlation functions obtained for noninteracting particles having diffusion coefficients in a large range form 10−12to 5×10−10m2/s are very well fitted by the theoretically expected hyperbolas.