The simultaneity of the presence of substrate (inducer) and the absence of a better nitrogen nutrient causes a strong cooperative effect (catabolic synergism) on arginase production. This effect is shown to operate by a specific mechanism. carg A+ 0h mutation (Dubois et al., 1978) identifies an element of this process located near the arginase structural gene and acting in cis. This mutation produces constitutivity for synergism in addition to constitutivity for induction (this last effect is produced alone by cargA +0- operator constitutive mutation). The receptor of the signal for the presence of substrate is the same as for induction. cargA + 0h mutation allows to make further distinction between the promotion of arginase synthesis caused by nitrogen limitation and nitrogen starvation.