Fallout3: Not gaining experience points.?

GNR building - many supermutants and one super mutant behemouth - after killing the normal ones, and the behemouth I gain no experience. I was with Charon the first time, so I loaded the game and left Charon in Megaton and did it all over again myself - still not gaining any experience from it...anyone tell me why? Am I not supposed to gain exp for this part, as BoS members are helping you kill it?