My Oshawott didn't get any exp from defeating two wild patrats, so...
If you are using emulators, unfortunately there is a glitch in the Black/White ROM in which no Pokemon ever gains EXP.
This is a known glitch in several emulators and is there so that people who want to free-load the game have a hard time.
Source: Have seen it in action, experience.
Hope I helped!
Well if your playing a DS game then you might have gained a small amount of exp and didn't realize it. Otherwise it's probably a glitch and you may have to restart your game. If your playing on a ROM, then this post might help you: http://www.gamefaqs.com/ds/995081-pokemon-white-version/answers/282187-not-gaining-exp
Source: Knowledge and the link above^