After seeing its opening performance Monday, I cannot help comparing the Sadler's Wells Theatre Ballet with its parent group which came to Boston last year. Although the former does not boast any prima ballerinas, it is far from second rate. Perhaps its is the corps' youthful enthusiasm and lively imagination which inspires their lithe, confident movement. The male dancers are especially agile and graceful, and the group in general shows versatility seen too infrequently in classical ballet.

In the Casse Noisette (the revised Nutcracker Suite) the dancers' spirit combines with magnificent costuming to create a delightful fairyland. As originally choreographed by Ivanov, this ballet was rather pointless and uninspiring. Frederick Ashton's revision retains the more colorful scenes, includes a pas de deux, and makes the myth more enjoyable. The finale of Tchaikowsky's Suite, however, assigns too much force to the Kingdom of Ice scene, causing Pauline Harrop and Robert Lunnon to put excessive power into their gestures. A necessary and remarkable scenery change to the Kingdom of Sweets also demands awkward pushing motions.

Once in the Kingdom of Sweets, nevertheless, the corps dances with brilliance and precision. Elaine Fi-field as the Sugar Plum Fairy has beautiful poise, and the Nutcracker Prince, David Blair, performs his ballon impressively. The divertissements--Chocolate from Spain, Tea from China, and Nougat from Russia were marvelous caricatures.

The modern dance selection, Harlequin in April, is a Boston premiere. Slightly reminiscent of the Faust legend, Harlequin represents "human aspiration reborn with the flowers in April. This aspiration makes it possible for him to break away from the plants, who are earthbound and self-sufficient." As the curtains draw back, you see Pierrot, the fool, garbed in a baggy white robe. Using magic, he transforms what looks like a mound of objects into humans who writhe as it in Dante's Interno. The mystic mood is extremely moving, especially when Pirman Treeu battles with an ever-increasing host of unicorns to win Columbine. Treeu is one of the most powerful and graceful dancers that I have seen in this medium. Praise should also go to John Cranko for the fresh, wierd choreography and to John Piper for effective costumes and decor.

Finishing the evening in their native element, the Theatre Ballet put on Pineapple Poll--another premiere--adapted from Gilbert's Bab Ballad. True to the Gilbert and Sullivan formula, tragedy and comedy have play, in this case on board the H.M.S. Hot Cross Bun. Elaine Fifield shows great talent for the comically forlorn gesture in her futile attempt to attract the Captain (David Blair), who keeps a crew of ladies in disguise.

Especially in those dances other than classical ballet," the Sadler's Wells Theatre group is extremely capable and enjoyable. The polished execution and coordination of the corps as a whole makes up for what it may lack in individual excellence.