Sprint Nextel Corp. said Monday it would acquire Alamosa Holdings Inc., its largest Sprint-branded wireless affiliate, for $3.4 billion.

Alamosa shareholders would receive $18.75 per share, a 15 percent premium from the stock’s Friday close of $16.26. The deal also includes assumption of $900 million in debt.

Alamosa shares gained $2.09, or almost 13 percent, to close at $18.35 in trading Monday on the Nasdaq Stock Market. Sprint Nextel shares rose 23 cents to close at $25.17 on the New York Stock Exchange.

Alamosa, based in Lubbock, Texas, sells Sprint-branded services in 19 states and has 1.48 million subscribers. Sprint Nextel expects to close the deal in the first quarter of next year.

Alamosa is the fourth affiliate Sprint Nextel has acquired since Sprint Corp. bought Nextel Communications Inc. in August.