DALLAS — Southwest Airlines posted net income of $608 million in the second quarter, its highest quarterly profit ever. The parent companies of United Airlines and Alaska Airlines also reported record profits for any quarter.

Southwest topped its previous best quarter of $465 million, set in the second quarter last year.

Dallas-based Southwest saw its fuel bill drop $420 million to just over $1 billion, offsetting other increases in expenses. Its revenues climbed $100 million to $5.1 billion.

Excluding special items, its net income reached $691 million, or $1.03 a share. Analyst consensus had been that Southwest would earn $1.02.

The company saw its unit revenues – revenues per seat per mile – drop 4.7 percent in the second quarter. However, CEO Gary Kelly said Southwest expects unit revenues in the third quarter to be off just 1 percent.

“We continue to be extremely pleased with our development markets in Dallas,” Kelly said. “They are remarkably strong, surpassing system average margins and returns.”

United Continental Holdings Inc., parent of United Airlines, also reported record earnings for the second quarter when it released its results Thursday. In fact, United said it was its best quarter ever.

The company earned $1.2 billion in net income, or $1.3 billion excluding special items. Its revenues dropped $415 million to $9.91 billion in the quarter, but its fuel bill declined even more, down $995 million to $2.11 billion.

United also announced that it would launch a $3 billion repurchase of its own shares. It expects to complete the stock buybacks by the end of 2017.

Alaska Air Group, parent of Alaska Airlines, also announced its best quarter ever with net income of $234 million, or $230 million ex-items.