Materials in the experiments were prepared by absorbing tb.e extracts of fish food N-Paraffin yeast. They were tested,ar znu enic properties by using the Sal.monallaTyphimurium microsomal test system. Mutagenic activity was detected in the prepared N-Paraffin yeast materials. It re-eals that this fish food may contain the compound of flameshift and base.-pair substitution mutagens. It required metabolic activations. Therefore great care should he taken in using this yeast as the food of fish or other animals in order to av aid the potential harmfulness.