U.S. Cellular and Alltel Wireless plan to sell a co-branded prepaid service at Walmart stores in 18 states beginning in May. 

The tie-up could help postpaid-centric U.S. Cellular gain a larger presence in the no-contract market. 

"Teaming with Alltel to gain distribution in Walmart gives us another opportunity to be where our customers want to shop," Carter Elenz, U.S. Cellular executive vice president of sales and customer service, said in today's announcement.

The U Prepaid service will run on U.S. Cellular's network or Alltel's network depending on customers' location. The companies did not list states slated for U Prepaid or provide exact launch dates.

Four phones from Samsung and LG will be offered with U Prepaid at launch, including the Samsung Repp, Samsung Chrono, LG Saber and LG Attune/Exchange. 

The companies did not disclose specific pricing plans for the service, but said the plans would be "nationwide with no roaming charges" and would include multiple data options and plans that include unlimited talk and texting. 

U Prepaid will compete at Walmart with a number of other prepaid offerings, including Tracfone's StraightTalk, AT&T's GoPhone, Cricket and T-Mobile USA.