getArgument es NULL, ¿por qué?

I am trying to pass values through a Bundleusando setArguments() from an activity to another class extends Fragment when I press one of the listed items in a ListView.

As shown below, I used a Log.i statement to know if the Bundle received is null or not. Unfortunately, it is always null and consequently, no data shows on the designated TextView of the class that extends Fragment. What I am missing or what is wrong with the code.

FragmentClass:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    // TODO Auto-generated method stub

    View view = inflater.inflate(R.layout.fragmentclasslayout, null);
    return view;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onActivityCreated(savedInstanceState);

    if (getArguments() != null) {
        Log.i(TAG, "getArgument is not null");
        int value = getArguments().getInt("pos");           
        TextView tv = (TextView) getView().findViewById(R.id.tvID);
        tv.setTextSize(value);
        tv.setText("size = "+value*10);
    }else {
        Log.i(TAG, "getArgument is null");
    }
}

MainClass_implements_onItemSelectedListener:

setContentView(R.layout.mainactivity);

    ListView mListView = (ListView) findViewById(R.id.list);
    ArrayAdapter<String> mArrayAdapter = new ArrayAdapter<String>(getApplicationContext(),
            R.layout.listviewlayout, R.id.tv, string);
    mListView.setAdapter(mArrayAdapter);
    mListView.setOnItemSelectedListener(this);
}

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position,
        long id) {
    // TODO Auto-generated method stub

    Fragment f = new FragmentClass();
    Bundle bundle = new Bundle();

    bundle.putInt("pos", (position+1));
    FragmentClass fc = new FragmentClass().newInstance(bundle);
    //f.setArguments(bundle);

    FragmentTransaction t =
            getFragmentManager().beginTransaction();
            t.replace(R.id.fragment00ID, f);
            t.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
            t.addToBackStack(null);
            t.commit();
}

mainActivity_layout:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="horizontal"
tools:ignore="MergeRootFrame">

<ListView
    android:id="@+id/list"
    android:layout_width="100px"
    android:layout_weight="1"
    android:layout_height="match_parent" />
<fragment
    android:name="com.example.fragmentwithlistview.FragmentClass"
    android:id="@+id/fragment00ID"
    android:layout_toRightOf="@+id/list"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent"
    android:layout_weight="4" />

FragmentClass_layout:

<FrameLayout 
xmlns:android="http://schemas.android.com/apk/res/android"    
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
tools:ignore="MergeRootFrame"
android:background="#FFFFFF">

<TextView
    android:id="@+id/tvID"
    android:textColor="#000000"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"/>