Inaccessible due to protection levels

Hey,

I'm currently doing some work on a program i'm creating for Uni, now i'm getting the above error on the following line of code:

if ((position.X < boundary.Width - 50) && (counter == 0))

All the variables are set to Public. The error is within an 'Alien' class but the error message says this: ".enemy.counter is inaccessible due to protection levels"

I have embolded where Visual Basic 2008 is locating the error. I can upload more code if required.

Any help would be really appreciated!!!

Cheers

P