In this paper the authors report about the results of five tests using phosphorous fertilizers on cotton, these tests carried out in several soil unities of the State of São Paulo. The effects of two fertilizers were compared, i.e., plain superphosphate and fosforita de Olinda, each applied by itself or in mixture, at time of planting. As a result, it could be stated that plain superphosphate excelled fosforita de Olinda when they were applied isolatedly. A partial replacement of superphosphate by "fosforita" frequently caused a reduction in the yield, although in one only case yield differences showed to be statistically significant.