The following two behavioral experiments were conducted in a small water tank (90×45×30 cm) to examine the characteristics of activity of juvenile walleye pollock. 1) Locomotor activity was observed with an infrared ray sensor under the different light-dark conditions, 12 h light (100 lx) and 12 h dark (<0.1 lx) cycles (LD), 24 h constant dark (DD) and 24 h constant light (LL). 2) Fish behavior to net under different illuminance (0.1, 1.0, 100 lx) was recorded by a CCD camera. The activity of juvenile walleye pollock showed clearly a diurnal rhythm under the LD condition, which gradually decreased after 18：00 and increased after 6：00. However, the particular rhythms such as circadian rhythm were not shown in other conditions. The activity of juvenile walleye pollock was considered to largely depend on illuminance condition. In the experiment with relation to behaviors toward a net, the linear regression coefficient between the cumulative number of passing through the net and elapsed time was nearly constant with various illuminance, while the regression coefficient of the approach to it increased with illuminance. The passing rate, the coefficient of passing/the coefficient of approach decreased gently as illuminance increases. The probability of capture for juvenile walleye pollock by a mid-water trawl was estimated separately at day and night based on the method of Barkley.