Problems with adding multiple conditions

I have an If statement that will make the player jump once, I don't see what I am doing wrong at the moment. Here is the code:

 #pragma strict

 var jumpforce : float = 50.0;
 var isgrounded : boolean = true;

 function Update () {

     if(Input.GetKeyDown("space") && isgrounded = true){
         rigidbody.AddRelativeForce (0,jumpforce ,0);
         isgrounded = false;
     }

 }

I would be grateful if anyone could point out my mistake :)

Thanks in advance.