A turbidity measuring device having a four-beam, alternating light arrangement for registering turbidity of a measured medium includes first and second light sources L1, L2; and first and second receivers R1, R2. The direct measuring paths extend from light sources Li, through a measured medium, to receivers Ri, and indirect measuring paths extend from light sources Li, through the measured medium, to second receivers Rj; wherein i≠j; wherein turbidity can be ascertained as a function of a quotient A/B by means of an evaluating circuit; wherein A and B are functions at least of signals registered via the direct or indirect measuring paths; wherein at least a first monitor signal, which depends on the first light source, enters into one of the two terms A or B; wherein the light reaches the monitor from the first light source without interaction with the measured medium; and wherein the monitor signal is added to at least one of the signals registered via the measuring paths and entering into the term A or B.