Operation of a cw thulium laser emitting at 816 nm has been demonstrated in bulk Tm:YLF with 46% slope efficiency. Prior cw demonstrations of this transition have been limited to ZBLAN fiber hosts and prior lasing in bulk crystalline host material has been limited to quasi-cw operation due to population trapping. Trapping at the 3F4 level was mitigated by co-lasing at 1876 nm. The co-lasing technique should be applicable to room-temperature operation and to power scaling of YLF and other crystal hosts.