Some of the previously described malT-lacZ fusion strains (Débarbouillé and Schwartz, 1979) produce very low amounts of beta-galactosidase activity and hence grow poorly on lactose. Spontaneous mutants growing faster on lactose have been isolated. Some of the mutations map in, or close to, the promoter of the hybrid gene. They lead to an increased production of the hybrid proteins, which then become detectable on polyacrylamide gels. This effect is cis dominant. When the mutations, called malTq, are transduced into a malT+ background the resulting transductants express the three maltose operons in a partially constitutive way. The malTq mutations therefore represent a new type of constitutive mutation. Their existence provides further evidence for the previously proposed model of positive regulation in the maltose regulon. In addition they should facilitate the purification of the malT product, and the identification of the malT promoter on the DNA.