Nighttime evapotranspiration (ETn) has typically been neglected in estimating water loss from land surfaces. Our objective was to quantify the contribution of ETn to daily (24-h) ET24 of irrigated and dryland cotton (Gossypium hirsutum L.) and irrigated alfalfa (Medicago sativa L.) grown in a semiarid climate. Nighttime ET was measured at Bushland, TX using weighing lysimeters containing monolithic soil cores of a clay loam soil for alfalfa in 1998 and cotton in 2001. The ETn of alfalfa averaged 6.9 % of ET24, with losses exceeding 1 mm occurring on at least 10 nights, with a maximum single night loss of 1.94 mm. Average ETn of cotton contributed from 3 % under dryland conditions to 7 % under fully irrigated conditions. The largest single ETn event for cotton was 1.24 mm, with events exceeding 1 mm occurring four times. Nighttime ET can be an important part of total daily ET of irrigated drops in a semiarid environment.