We report on the sub-Doppler laser cooling of neutral $^{171}$Yb and $^{173}$Yb in a magneto-optical trap using the $^{1}S_{0}$-$^{1}P_{1}$ transition at 398.9nm. We use two independent means to estimate the temperature of the atomic cloud for severa
l of the Yb isotopes. The two methods of MOT-cloud-imaging and release-and-recapture show consistency with one another. Temperatures below 400$mu$K and 200$mu$K are recorded for $^{171}$Yb and $^{173}$Yb, respectively, while ~1mK is measured for both $^{172}$Yb and $^{174}$Yb. By comparison, the associated 1D Doppler cooling temperature limit is 694$mu$K. The sub-Doppler cooling of the I=1/2 $^{171}$Yb isotope in a $sigma^{+}-sigma^{-}$ light-field trap adds further evidence that the Sisyphus cooling mechanism is occurring in such 3D magneto-optical traps.
