Tyrosinase derepression in Neurospora mycelia grown in Vogel medium, submitted to starvation in phosphate buffer 0.1 M, pH 6.0, was abolished by exogenous magnesium sulfate. This effect seemed to be caused by the sulfate ion itself and not by a sulfate-derivative. Sulfate repression required protein synthesis, thus suggesting the involvement of a specific gene product mediating sulfate repression. Cultures made in Westergaard and Mitchell crossing medium became competent for sexual development and could be stimulated to form tyrosinase either by mating or starvation. In that case the enzyme derepression was insensitive to the sulfate effect. The possible existence of a positive mechanism for the control of tyrosinase activity during sexual development is suggested.