Incomplete stomatal closure at night can result in substantial water loss at times when photosynthetic carbon gain is not occurring in C3 and C4 plant species. To investigate the magnitude of nighttime water loss for a crop species in the field, measurements of nighttime water loss by tomato (Lycopersicon esculentum Mill. cv. Heinz 8892) were made by three methods: a field-scale lysimeter and two leaf-level instruments, an automated viscous flow porometer and a portable photosynthesis system. The portable photosynthesis system indicated nighttime transpiration of 10% of maximal daytime transpiration and the viscous flow porometer demonstrated partially open stomata. Integrated crop water loss during the dark, non-photosynthetic hours measured on the lysimeter was 3-10.8% of total daily water loss. In the glasshouse, a survey of closely related wild and cultivated tomato species showed that under ambient conditions nighttime transpiration varied within and among species and was 8-33% of maximal daytime transpiration. Implications of such a substantial fraction of total daily crop water use occurring during the night are significant in agronomic, environmental, and economic terms. Further, variation within and among species in nighttime water loss has implications for breeding to improve crop water use efficiency.