Roberts, the MW record holder in the men's 100-meter butterfly, placed 21st among 122 swimmers in the prelims of the event with a time of 54:01.
Roberts' teammate, senior David Seiler, was close behind, posting a time of 54.33 to finish 35th.
Meanwhile, UNLV senior Dannica Brennan placed 102nd in the prelims of the women's 200-meter backstroke, finishing in 2:18.52.
The swimmers with the top 16 times in the preliminary heats advance to the semifinals. From there, the top eight finishers in the semis move on to the finals, with the first- and second-place finishers qualifying for the 2012 Olympics in London (July 22-August 12).
Boise State's Amber Boucher and Stephanie North will be joined in the prelims of the women's 50-meter freestyle today by UNLV junior Rachel Dixon and former Wyoming standout Kelsey Conci. The trials conclude on Monday with finals in the women's 50-meter freestyle and men's 1500-meter freestyle.