Diel activity ofJesogammarus spinopalpus was monitored by an actograph using infra-red beams. The overall level and diel pattern of activity differed between males and females, and between the single and precopula phases. Single males were more active during the dark period (night) (153 intercepts/h) than during the light period (daytime) (50 intercepts/h), showing a clearly nocturnal habit, whereas the activity of single females was very low both at night (18 intercepts/h) and in the daytime (8 intercepts/h). The activity pattern of precopula pairs was nocturnal, but the level of activity was lower than that of single males (48 intercepts/h at night and 6 intercepts/h in the daytime). The sexual difference in activity level may reflect behavioral tactics during reproduction.