This study compared the forces of tooth separation generated by steel springs, NiTi springs, and elastomeric separators in vitro. Elastomeric separators gave the highest force levels and NiTi springs showed the lowest. When the teeth where separated incrementally 0.1mm at a time, NiTi springs showed a gradual drop of force, elastomeric separators showed a steep drop from their high initial forces to a relatively high remaining force at 0.8mm separation, while steel springs gave the steepest drop that fell below 50 grams at 0.3-0.5mm separation. In conclusion, NiTi springs gave moderate initial separation forces that remained effective even at 0.8mm separation.