This paper reports about an experiment carried out in argillaceous soil of lowlands, located at the Experiment Station of Pindamonhangaba, in the Paraiba River valley, and had in view the study of application of ammonium sulfate as top dressing, in rice crops. This same trial had further the purpose to find out which were the necessary doses of nitrogen that led to major production as well as the levels of phosphorus required for a good development and good yield of rice. The experimental lay-out used was that of ransom blocks where 5 doses of nitrogen were applied, 3 of phosphorus and one only of potassium. The variety of rice (Oryza sativa) used was the Iguape Agulha the culture being submitted to flood irrigation, which is the type common to the rice crops in that region of the State. The results obtained indicate the great efficiency toward increase of yield by application as top dressing of the ammonium sulfate. This fertilizer should be applied in the period between 30 and 50 days after germination. Responses determined by the doses of nitrogen were linear and significant, indicating that under the conditions of the trial, even the application of 120 kg per hectare of the element was still efficient for the increase of production. As for the phosphorous fertilizer no reaction whatever was observed.