This study was conducted at the Agronomic Institute of Parana (IAPAR), Londrina, Parana state (latitude 23° 18' S, longitude 51° 09' W, average elevation of 585 m). Londrina's climate in Koppen's classification is Cfa type, that is, humid subtropical climate with rain in all seasons and droughts in winter. Soil water evaporation (E) was determined for different soil mulching cover fractions. The treatments were applied on 2.66 m2 x 1.3 m deep weighing lysimeters for the determination of E by mass difference with an accuracy of 0.01 milimeter in 1 h intervals. Evaporation was determined for different mulching fractions using the average amount of wheat straw produced in that region of 5 ton.ha-1 distributed onto 25, 50, 75 and 100% of the soil surface. In the first cycle (28 October to 27 November, 2008), the reductions of E in treatments with 25, 50, 75 and 100% mulching cover in relation to the not-mulched soil were 15, 17, 20 and 30%, while in the second cycle (19 January to 16 February, 2009), the reduction were 15, 30, 45 and 60%, respectively. Key words: Wheat straw, mulching uniformity, soil water loss.