Assets/Scripts/PlayerController.cs(5,33): error CS0246: The type or namespace name `MonoBehavior' could not be found. Are you missing an assembly reference?

code goes as follows:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;

 public class PlayerController : MonoBehavior {

     public float speed;

     private Rigidbody rb;

     void Start ()
     {
         rb = GetComponent<Rigidbody>();
     }

     void fixedUpdate ()
     {
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
         rb.AddForce(movement * speed);
     }
 }