Effects of N and K applications on cotton fiber quality obtained in field tests conducted in latosolic soils of the State of São Paulo are related. Nitrogen fertilization increased fiber length and length uniformity, in soil with medium K content, and fiber strength in soil with high K content. The fiber characteristics were not affected by N application in soils with Potash ceficiency. Potassium fertilization increased length uniformity, Micronaire and maturity index, without changing the fiber length, in soil with low K content. The fiber characteristics were not affected by K application in soil with h:gh K content.