How to select childs located at the pointed nesting level?

How to select childs located at the pointed nesting level?

Today I have used JQuery first time and at one stroke bumped into problems.
How to select childs of the element located at the pointed nesting level?
For example, I have a menu tree:
<div>
<ul>
<li>
#1
<ul>
<li>#11</li>
<li>#12</li>
</ul>
</li>
<li>
#2
</li>
</ul>
</div>
How could I select all "ul"-element at the second level of nesting; is it possible?