Hollywood Casino Perryville, the larger of the two, brought in some $784,000 less than in May.
The decline at Perryville was offset in part by revenue increases at the smaller Casino at Ocean Downs, which took in nearly $3.78 million last month, up from $3.69 million in May.
Sales at the 750-machine Eastern Shore casino have steadily increased since it opened in January. Owner William Rickman told state racing regulators last week that revenue has been lower than anticipated.
State officials and analysts expect the casino near Ocean City to do its strongest business during the summer months, when more tourists are in town.
In May, the state's two casinos took in $13.29 million, down about 2 percent from April.
Since the Perryville casino opened last fall, the two casinos have generated $103 million in revenue. That money reflects sales after winners are paid out.
About half of the revenue, or $49.98 million, has been set aside for the state Education Trust Fund.
The casinos keep 33 percent, while the remaining money is split among several entities, including the Maryland Lottery, the horse racing industry, local government and small, minority- and women-owned businesses.