Substituting fish of lesser value for high priced fish such as grouper has become a major problem for restaurants, wholelsalers, and the general public. A method based upon nucleic acid amplification enables clear differentiation between grouper and potential substitutes. The nucleic acid is incubated with a molecular beacon probe thereby allowing detection of the hybridized molecular beacon probe. All grouper, including the fresh market sample, were successfully confirmed as grouper. Several non-grouper samples obtained fresh and frozen from the market were also tested; none were detected as grouper.