A wide variety of fish are known to induce allergic reactions following ingestion or inhalation of vapors by sensitized individuals. Although the exact prevalence of fish sensitivity is not known, fish are among the most important food allergens; and as consumption of fish increases, rates of sensitization are expected to increase. Diagnosis of fish allergy is aided by clinical history, skin prick testing, and in vitro assays; however, double-blind placebo-controlled food challenges are the most reliable method to confirm fish allergy and to identify putative species. It appears from RAST inhibition and SDS-PAGE/Western blot studies that the current policy of recommending that fish-sensitive individuals avoid all species of fish should be reevaluated. The major allergen in codfish (Gad cI) is one of the most extensively studied of all allergens. It is a calcium-chelating protein, with a mol wt of 12,328 kDa and an isoelectric point of 4.75. As an allergen, Gad cI is very stable and its allergenic activity appears to be dependent on amino acid sequence. Crossreactivity among some fish species may be the result of common structures within related proteins.