How do I tell my boyfriend I'm not comfortable with him being friends with someone?

My boyfriend's best friend is a female. Well actually most of his friends are females, but there is only one I have a problem with. I've always been told to be weary of the ones who don't try to befriend you personally. Every time they hang out I'm not welcome to come because of this female friend (my boyfriend declines every time unless I am invited) She always tell my boyfriend that he spends too much time with me and constantly tries to get him to go against agreements we have as a couple as far as alone time with opposite friends. How do I tell him that I am not comfortable with this particular friend?