The effect of nitrilotrimethylenephosphonic acid (NTMP) on calcium sulfate dihydrate nucleation has been studied at 25 degrees C in a batch crystallizer with a related optical device. This paper reports the measurement of the induction period done under different supersaturation ratios ranging from 2.60 to 4.99, while varying the NTMP concentration in the interval 0.005-0.1 g/L. The comparison of the interfacial tension values estimated in the presence of NTMP with respect to those previously obtained without any additive in the mother liquor and in the presence of citric acid, by using the nucleation-controlled mechanism, suggests that NTMP is more effective in retarding gypsum nucleation than citric acid. Besides, the examination by optical microscopy of the crystals formed indicates that NTMP also modifies the crystal habit of gypsum, resulting in less elongated platelike particles.