A step-indexed optical fiber is tapered to a diameter of one-third of the original diameter by heating under tension. Some of the cladding in the tapered region is removed by etching; and, this region is immersed in refractive index liquids in the range 1.33 to 1.65. The percentage of the light transmitted by the tapered section decreases as the index of refraction increases. Changes in index of refraction of 6x10-5 are measureable. The refractometer sensitivity may be optimized in the different index ranges by measuring the intensity in the various modal groups.