Electroabsorption (EA) measurements on ZnS crystals were performed. The electric fields were applied parallel to the polar c‐axis. In single‐crystal regions the EA was found to be independent of the sense of the electric field. On the other hand, in those crystals which contain various polytypes and stacking faults and exhibit an anomalous photovoltaic effect the EA was strongly dependent on the sense of the electric field with respect to the crystallographic polarity. From the spectral and field dependencies of the EA it was concluded that there exist in these crystals internal fields which are uniquely related to the crystallographic polarity. The sense of the electric fields in the various structures was determined. The strength of the fields was found to depend on the structure composition of the crystals; fields up to 105V/cm were estimated. The existence of opposing internal fields, as found in this work, verifies the assumption of several authors in explaining the anomalous photovoltaic effect in ZnS crystals.