Yes, it’s August. And if you’re not lying on the beach sipping drinks with little umbrellas in them, you’re wishing you were. But the lazy days of summer are no excuse for the overly casual work of the Mark Morris Dance Group on Thursday night.

The first piece on the program – “A Lake” – looked as if the dancers had all just rushed on stage after two weeks at a cabin beside one. Clearly, the Morris men and women were dancing for a classical music crowd. They wouldn’t dream of turning up like this for a BAM audience.

Created in 1992, “A Lake” is set to Haydn’s Concerto for Horn and Strings in D major, and it is built on a series of often transfixing patterns. There are some angular, well-drawn poses that contrast with lots of joyful swoops and runs.

Danced as it was, however, the look was flabby and entirely too easy-going. There’s something to be said for a “natural” feel to choreography – schlumpy arms and loose hands, that sort of thing. But even intentional softness needs a bit of polish and zip to take on the intended effect.

Things didn’t get much better in “Marble Halls,” which with its beachy costumes, seemed only to remind the audience that we could have all been somewhere sandy if we had only arranged it. Short, purple spandex pants were paired with orange tank tops that looked like something for the “Strongman” at Coney Island. There was even a repeated move in this piece that suggested diving or swimming: the dancers lunged forward, planted their hands down on the floor and brought their legs up.

This work – set to a Bach concerto – had the dancers jumping and kicking their heels up a bit. But it also brought them back to poses on which they rarely seemed to be hitting the same planes. The Morris dancers have very different body types, so they can’t really hit the same heights. But they could at least settle on the matter of degrees – should an arm be at a 45-degree angle or something a little wider, perhaps? Who cares? It’s August!

There did seem to be an attitude adjustment for the second portion of the evening. “I Don’t Want to Love” had a more deliberate, dramatic edge; it required more attention than horsing around the backyard. Marjorie Folkman seemed to take the lead in making the small stuff matter. Joe Bowie also turned up, looking solemn and grave as three dancers tried to distract him. And Lauren Grant was carried off stage looking like a perfectly placed angel.

The costumes in this work – designed by Isaac Mizrahi – suggest a leisurely evening in bed. There are short silk robes and long nightgowns for the women; the men wear pajama-type pants. It’s a look that com bines well with the abstract images of rejection and loneliness, as well as thoughts of love. The music – performed by the Waverly Consort – was a series of songs by Monteverdi.

The highlight of the program was “Jesu, Meine Freude,” set to the work by the same name by Bach and sung by Voices of Ascension. This piece benefited enormously by the dominating presence of dancer Matthew Rose.

The work starts with him on stage and another dancer behind him moving in a way that suggests we are looking into some sort of mirror. Mr. Morris’s choreography calls for slow, strong arm movements that start at the chest and radiate outward, as if the dancer was letting his light shine and reaching out to the world.

Mr. Rose performed this move with meticulous attention to detail, and seemed to treat everything a bit more seriously. He seemed to set the tone for the rest of the cast, for during this piece – the last – the company finally decided to show up.