  We measure the temperature of a deeply degenerate Fermi gas, by using a
weakly interacting sample of heavier bosonic atoms as a probe. This thermometry
method relies on the thermalization between the two species and on the
determination of the condensate fraction of the bosons. In our experimental
implementation, a small sample of 41K atoms serves as the thermometer for a 6Li
Fermi sea. We investigate the evaporative cooling of a 6Li spin mixture in a
single-beam optical dipole trap and observe how the condensate fraction of the
thermometry atoms depends on the final trap depth. From the condensate
fraction, the temperature can be readily extracted. We show that the lowest
temperature of 6.3(5)% of the Fermi temperature is obtained, when the
decreasing trap depth closely approaches the Fermi energy. To understand the
systematic effects that may in uence the results, we carefully investigate the
role of the number of bosons and the thermalization dynamics between the two
species. Our thermometry approach provides a conceptually simple, accurate, and
general way to measure the temperature of deeply degenerate Fermi gases. Since
the method is independent of the specific interaction conditions within the
Fermi gas, it applies to both weakly and strongly interacting Fermi gases.
