The Rams agreed to terms with defensive tackle Kendall Langford but lost Jason Jones to the NFC West division rival Seattle Seahawks on Saturday.
Langford, 6-6, 296, has agreed to a four-year, $24 million deal that includes $12 million in guaranteed money.
Langford, 26, played end in the Dolphins' 3-4 front but will play tackle in the Rams' 4-3 scheme. Prior to visiting St. Louis, he had visited Cincinnati and was also drawing interest from Green Bay.
Strong and powerful, Langford has been a four-year starter for Miami, which has had one of the league's top run defenses the past couple of seasons.
Despite reports to the contrary, even after agreeing to terms with Langford, the Rams still were trying to sign Tennessee Titans free agent Jones. (After releasing Fred Robbins and Justin Bannon earlier in the week, the Rams need two starting defensive tackles.)
But Jones signed instead with the Seahawks. He has had some knee issues, which may explain why he signed only a one-year deal with Seattle.
Meanwhile, another free agent who visited St. Louis over the past couple of days, former Carolina offensive guard Travelle Wharton, is signing with Cincinnati instead.
After agreeing to terms with former Green Bay center Scott Wells on Friday, the Rams probably couldn't afford to make another sizable free-agent investment in the offensive line.
Also, wide receiver Mario Manningham (New York Giants) was still conducting his Rams visit Saturday. The Rams will sign him if the price is right.