Income from the casino's operations for the second quarter was $72.4 million, down $3.9 million, or more than 5 percent, from the same period last year.
The decline was driven by a drop in slot machine revenue, which dropped $4.3 million to $223.4 million. Mohegan Sun does not report profits for slot machines.
The decline was due to the expansion of slots at Yonkers Raceway in New York and Twin River in Lincoln, R.I., casino executives said.
In addition, 260 fewer slot machines operated at Mohegan Sun compared with the same quarter last year.
"Clearly, this quarter we learned a lot about the Northeast market," said Jeffrey Hartmann, executive vice president and chief operating officer.
The casino also is reassessing the cost of its $740 million expansion project, which includes more casino space, stores and restaurants, and a new hotel tower.