We report the results of optical reflectivity measurements on MBe13(M=U,Ce,Th) from 0.1 to 30 eV at T=300 K. The dielectric function ()=1+i2() is determined from a Kramers-Kronig analysis of the data. The intraband and interband contributions to () were separated via a low-energy Drude analysis. The following free-carrier plasma frequencies were obtained: p(UBe13)=5.9 eV, p(CeBe13)=5.3 eV, and p(ThBe13)=4.4 eV. Estimated values for the optical mass of the free carriers fall in the range 1