The present study examined the effects of visual cues provided by a reflective environment on spontaneous behavior exhibited by crayfish (Procambarus clarkii) in an empty aquarium. The predominant components of spontaneous behavior were determined by observing the activity of solitary adult crayfish for 20 minutes in a glass aquarium containing fresh water and no objects. Five distinct behaviors were observed: rearing up (climbing on the wall), turning, cornering (facing the corner for 5 s or longer), backward walking, and crossing (crossing the midline of the aquarium). The frequencies of rearing up, cornering, and turning decreased when reflection from the glass wall was blocked with black cardboard or non-reflective plastic. In a tank containing mirrors on one side and non-reflective plastic on the other, crayfish cornered, reared up, and turned more in front of the mirrors. To examine whether or not such responses depend on socialization, crayfish were housed for two weeks either in same-sex pairs (socialized) or separately (isolated), and subsequently their behaviors were compared in a tank with mirrors on one side and non-reflective plastic on the other. Crayfish that had been housed in isolation showed no difference in rearing up, turning, cornering, or backward walking between the mirror and non-mirrored portions of the tank. Crayfish housed in pairs showed significantly more of all five behaviors in front of the mirrors than in the non-mirrored portions of the tank, and they spent significantly more time in front of the mirrors than in the non-mirrored part of the tank. On the other hand, isolated crayfish seemed to avoid the mirrored side. The results demonstrate that crayfish are sensitive to reflection, even the partial reflection present in an ordinary glass tank, and that responses of crayfish to a reflective environment depend on prior socialization.