Optical frequency measurements of the intercombination line (6s2)S01-(6s6p)P13 in the isotopes of ytterbium are carried out with the use of sub-Doppler fluorescence spectroscopy on an atomic beam. A dispersive signal is generated to which a master laser is locked, while frequency counting of an auxiliary beat signal is performed via a frequency comb referenced to a hydrogen maser. The relative separations between the lines are used to evaluate the P13-level magnetic dipole and electric quadrupole constants for the fermionic isotopes. The center of gravity for the P13 levels in Yb171 and Yb173 are also evaluated, where we find significant disagreement with previously reported values. These hyperfine constants provide a valuable testbed for atomic many-body computations in ytterbium.