Opera Caslinga, Montgomery County's new opera company, was off to a promising start last week with its first production, Mozart's "Cosi Fan Tutte," at Kennedy High School in Silver Spring. Despite its limitations, the production, which will be repeated at 8 p.m. tomorrow and Sunday, speaks well for the future of the company.

One of Mozart's last three operas, "Cosi Fan Tutte" contains some delightful music. Its libretto, easily followed in Casalinga's English version, is no more ridiculous than most comic opera plots. Through various ruses, Don Alfonso, a good-natured cynic, proves to his friends Ferrando and Guglielmo that their sweethearts are as frail as all women when it comes to matters of love.

The Casalinga production eliminated the opera's few choral passages and substituted a piano for the orchestra. The staging is simple, set off by a handsome backdrop and highly effective costumes. The fine ensemble blend of the six-member cast testifies to the lengthy four-week rehearsal period.

The glory of the cast is tenor Christopher Leo King in the role of Ferrando. Apart from softer passages which lose all intensity, his rich tenor voice produces lines of thrilling warmth and smoothness. Robert Patton brings a delightfully mobil face and solid bass voice to his portrayal of Guglielmo. Though plagued by occasional pitch problems, Jed Collard, as Don Alfonso, contributes an intelligent musical and dramatic portrayal.

Among the women, soprano Judith Bauden combined a properly light vocal quality with a polished characterization to bring the maid Despina to impish life. Ellen Lang's dramatic mezzo-soprano was at times too heavy for the role of the fickle Dorabella but it was sensitively used. As the other fickle woman, Fiordiligi, soprano Susan Kincaid was included to shout and pout, which interfered with the expressive flow of her part. The cast as a whole needed to explore the lower end of the dynamic scale and introduce a lighter quality into its singing.

Music director Marc Tardue skillfully handled the piano accompaniment and stage director Terry Glaser generally found effective ways to underline the comedy without detracting from the thrust of the music. For details on this weekend's tickets call 946-8997.