Irrigation frequency is one of the major factors required for adequate irrigation control in soilless culture. In order to investigate the effect of irrigation frequency on transpiration, growth, fruit yield, and water use efficiency, the paprika plants (Capsicum annuum L.) were compared under different irrigation frequencies based on solar radiation in soilless culture systems. The plants were grown on rockwool slabs following the vertical trellis "V" technique. Irrigation started whenever cumulative solar radiation reached the set value. Two set values of 120 $J{cdot}cm^{-2}$ (high irrigation frequency, HIF) and $J{cdot}cm^{-2}$ (low irrigation frequency, LIF) were applied from 25 days after transplanting. Irrigation amount was controlled to keep a drain ratio at 20-30% of the total supply in order to avoid the salt accumulation in the root medium. Total water amount supplied to the plants in LIF was 94% of that in HIF. Transpiration in LIF or HIF was similar to that of estimated transpiration by Penman-Monteith equation, but slightly lower or higher value was observed, respectively. Leaf area and marketable fruit yield were not affected by the irrigation treatment. The both ratios of total transpiration to marketable fruit yield and total irrigation to marketable fruit yield were a little higher in LIF than in HIF. We concluded that water use efficiency in HIF was considered to be similar to or a little higher than that in LIF, but irrigation frequency did not affect the growth and production of paprika plants in open-loop system.