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A fiber optic fluorosensor which shows a specific selectivity for Ca2+ and Eu3+ is developed. This sensor uses a fluorescein-labeled calmodulin (F-CaM) as a sensing reagent which forms a fluorescent chelate with Ca2+ and Eu3+. A dialysis membrane is employed to entrap 0.50 mM EGTA/5.0 mM Tris-HC1 buffer solution (pH 7.0) with F-CaM at the common end of a bifurcated fiber optic bundle. The sensing mechanism of this sensor is based on the change in fluorescence intensity of metal-calmodulin complexes where CaM undergoes a conformational change upon binding with Ca2+ and Eu3+. The resulting change in fluorescence signal can be related to the concentration of Ca2+ and Eu3+ and used for determination of those metal ions. The detection limits of Ca2+ and Eu3+ are measured to be 5 × 10-8 M and 1.0 × 10-15 M, respectively. The addition of EGTA to the sample buffer solution minimized the effect of background calcium. The sensor showed the best response to F-CaM in pH 7.0 Tris-HCl buffer. The dialysis membrane of the higher molecular weight cutoff showed a better condition on the response characteristics of the sensor. The interference effects of Mg2+, K+ and Na+ were not significant to the sensor signal of Ca2+ and Eu3+.