I was with a man (never married) for 6 years. I had two children from a previous relationship. (ages 9/7) The minor children do not know they are not biologically his. We have separated and have began our lives apart. My question is can I still go after him if they are not biologically his? (Their biological father has never been around.)

Where is their father? Has his parentage been legally established?

So, he was your boyfriend? Not married, and not the father of your children? I'm no lawyer (for real, I am NOT a lawyer... ), but I would think that if he never adopted them then he would have no responsibility to do anything financially for any of you. (I would hope that he would still be involved with the children emotionally though... break-ups can be tough on them too... )

Once again, NOT a lawyer, just handing out what I see as some common sense...

In most cases the answer will be no. Why should a legal stranger take financial responsibility for the children when their father will not. He may continue to help you support them because of his relationship with the children, but is not legally obligated to do so. As I asked before, where is their father? Have you pursued a support order against him?

There biological father and I were only together for a few months long enough to get pregnant. We broke up cause I was pregnant and do to bad judgement he came back into my life and again I got pregnant. And as soon as I got pregnant the second time it ended. He wanted no children ( I have had my tubes tied since then and have matured enough to know I don't want a bunch of kids I can't take care of) and he disappeared. He is working under the table jobs and I can not locate him. Never filled tax returns and actual owes tens of thousands in back child support for another child he had. (previous to me) I don't even know what state he resides in, truthfully I can't tell you if he is alive or dead. My ex fiancé and I filled our taxes together as married, resided together for 6 years, we have debt together, and our children don't know of any other man, they call my ex fiancé dad.

Where did you live with the father? What State? (Not where he lives now, where he lived with you and the children.)

Missouri and that was 7 years ago.

The bio father should be the one paying support. Is there a reason you haven't requested it from him ?

My question is can I still go after him if they are not biologically his? (Their biological father has never been around.)

You cannot!
Even if you were married, you would be out of luck, too.
210.826, and 210.828 and a section 210.854 allow men who have been declared the father of a child by a court to petition to set aside the judgment and obtain relief from child support obligations when DNA testing shows that they are not the biological father,