The relative efficiency of various phosphorus fertilizers for com has been tested in lhe Instituto Agronômico for many years. In this paper the authors report the results obtained in the first series consisting of six experiments started before 1941. As a whole, ordinary superphosphate and Rhenania phosphate gave similar results and were superior to Thomas slag, bone meal, Serranaphosphate, Cibraphosphate and Rophos-phate. Granulated superphosphate, which was included in two experiments, was a little superior to the ordinary product in one and much inferior in the other. Rofoscal, studied in one trial, did not increase the yield. The authors warn, however, that the small number of experiments and the conditions under which they were conducted do not as yet permit to draw definite and detailed conclusions. This will possibly be done after reporting the next series of experiments.