Carl Icahn, on Tuesday, announced that he has a substantial stake in Southwest GasThis activist investor wants the Las Vegas-based utility firm to abandon its planned acquisition of Questar Pipeline, a natural gas company.

SWX management has committed a series of grave errors over the years at shareholders’ expense. Icahn sent a letter to Southwest’s board stating that Questar will be purchased for the amount you want. made public on Tuesday “The acquisition will lead to a serious decrease in shareholder value.”

Midday trading on Tuesday saw Southwest shares rise more than 5%. Icahn also stated that the company had not responded to his request regarding this matter in his letter.

CNBC did not immediately get a response from Southwest to its request for comment.

Icahn sent the letter after Reuters reported Southwest had been in advanced negotiations to acquire Questar (Dominion Energy’s transportation- and storage) business.

Warren Buffett’s Berkshire Hathaway wanted to pursue Questar last year. However, the deal was canceled in July because of “ongoing uncertainty associated achieving clearance by the Federal Trade Commission.”

Icahn addressed the letter to Southwest board and stated that he is “extremely dissatisfied” at management’s performances over the past several years as a “large shareholder”. His reasons included a poor relationship with regulators, runaway costs and a weaker credit profile.