A biker has serious injuries after taking an unexpected ride on the back of a semi-trailer truck west of Calgary.

R-C-M-P say the 37-year-old man and the semi were eastbound on the Trans-Canada Highway last evening when the motorcycle ran into the back of the big rig.

The man was flipped onto the rear fender of the semi where he rode for about half a kilometre before a motorist alerted the truck driver that he was carrying some extra cargo.

The biker is in a Calgary hospital and his injuries are considered life-threatening.