Carbon monoxide in imported tilapia was determined with a gas chromatograph equipped with a molecular sieve 13x column (2.3 m), a methanizer and a FID for the inspection of imported food. The concentration of carbon monoxide in the sample, which was vacuum packed and suspected to be treated with carbon monoxide, was 16 x 10 micrograms/kg in fish meat, and 37 x 10(3) microliters/l in the bubble in the package. On the other hand, carbon monoxide in the reference, which was vacuum packed but was not treated with carbon monoxide, was 10 micrograms/kg in fish meat and 76 microliters/l in the bubble in the package. Carbon monoxide was less than 4 micrograms/kg in two vacuum packed fish meat of tilapia sold in a market in Tokyo. From these results, the suspected sample was concluded to be treated with carbon monoxide for color fixating of protoheme in fish meat.