And there is a home debut for 20-year-old flanker Iain Henderson, who packs down at blindside, with Stephen Ferris switching to number eight instead of the injured Nick Williams.
Ulster, last season's Heineken Cup runners-up, are otherwise unchanged from the side that reeled off a fifth successive league victory of the campaign by defeating Connacht 25-0 six days ago.
"I can't wait," Henderson said.
"My first aim for the season was to be involved in a few PRO12 games, but with the few injuries that we have in the back-row I have been given an opportunity. I have to try to grab it with both hands."
Ulster have won 11 successive Heineken Cup home games, but French opponents Castres can claim just one tournament victory on the road from their last 17 starts.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
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The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.