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. Management of electrical conductivity (EC) is very important for crop growth in hydroponics. To determine the effect of EC on growth and yield of pak-choi (Brassica campestris ssp. chinensis) with growth stage, fresh and dry weights, number of leaves, yield, crop growth rate, and yield per area were evaluated in deep flow technique and nutrient film technique systems under different EC conditions. The growth stages were divided into two stages: stage 1, from seedling emergence to transplanting with 3 or 4 true leaves; stage 2, from transplanting to harvest at fresh weight ³60 g. The EC conditions were 0.5, 1.0, 1.5, 2.0, and 2.5 dSám1 for growth stage 1 and 1.0, 1.5, 2.0, and 2.5 dSám1 for growth stage 2. In growth stage 1, shoot fresh weight was highest at 1.5 dSám1, while shoot dry weight was highest at 2.0 dSám1. Both shoot fresh and dry weights were lowest at 1.0 dSám1. In growth stage 2, leaf length, leaf width and number of leaves were highest at 2.0 dSám1. Both shoot fresh and dry weights were highest at 2.5 dSám1. Yield per area was highest at 2.0 and 2.5 dSám1. Except for 1.0 dSám1, no significant differences in shoot fresh, dry weights and yield per area were observed. Cumulative water absorption per plant was highest at 2.0 dSám1. Adequate ECs for growth and yield of pak-choi were 1.52.0 dSám1 from seedling emergence to transplanting and 2.0 dSám1 from transplanting to harvest. Optimum conditions of EC could improve the growth and yield of hydroponically-grown pak-choi.