Improving water use efficiency requires the development of satisfactory means to evaluate plant water use in the field. One such method is a portable chamber for measuring crop water use on field plots. The objective of our work was to compare short term alfalfa (Medicago sativa L.) evapotranspiration (ET) measured with a portable chamber (CET) with that measured by a weighing lysimeter (LET). Intensive portable chamber measurements were made and microclimate data collected between 0500--2100h on 2 July 1980 at the meteorological station on the University of Minnesota Campus at St. Paul. The soil was a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed mesic Typic Hapludoll). Potential evapotranspiration (PET) was calculated using a modified combination equation of van Bavel. Expressed on an hourly basis, there was reasonable agreement between diurnal patterns of CET, LET and PET, although CET was as much as 0.16 mmh -1 lower than LET at ll00h and as much as 0.09mmh -l higher than LET at 1800 h. Daytime ET values between 0500 and 2100 h were 7.97, 7.71 and 7.58 mm for LET, CET and PET, respectively. The reasonable agreement between the CET and LET throughout the day and the daytime ET suggests that the chamber is satisfactory for measurements of crop water use on field plots.