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Florida's rapidly increasing population requires the efficient use of natural water resources. Accurate determination of reference evapotranspiration (ET o ) is essential to the estimation of a key water balance component. Data collected from automatic weather stations can be used to determine ET o following standardized methods, such as ASCE-EWRI. Net radiation (R n ) is one of the most important parameters in ASCE-EWRI. To evaluate the impact of net radiation on ET o , two weather stations were setup in large Bahia grass fields ( Paspalum notatum ) in central Florida. Meteorological data were collected at 15 min and 30 min intervals for ET o calculation. Daily average values were used in the analysis. The data were used to examine the impact of R n determined by three different methods, R n measured (R n-meas ), R n estimated from solar radiation (R n-EWRI ), and R n simplified from Irmak et al. (2003a) (R n-Irmak ). ET o calculated from R n-EWRI and R n-Irmak agreed very well with one another. ET o calculated from R n-meas was substantially lower than the other two methods. The R n-meas was 10–30% lower than the estimated R n-EWRI . This resulted in 6–20% lower ET o if using the measured R n . This suggests that either the use of the ASCE-EWRI R n equation in a humid region is questionable or the net radiometer needs to be examined.