Habituation to a novel environment (cross-shaped experimental chamber filled with water) was investigated in the crayfish Procambarus cubensis whose locomotor activity was recorded by a noninvasive “optical” technique. In the course of ten 30-min sessions, which were repeated twice a day for five days, the duration of an initial freezing reaction decreased, whereas there was no decrease in locomotor activity. The crayfish exhibited, however, a reduction of locomotor activity in the second session of the day, but this reduction did not carry over to the first session of the next day. In a second series of experiments, crayfish were placed in the same chamber for a single 5-hour session. As the session progressed, locomotor activity decreased to a level where animals were completely immobile. Tested the next day, these animals demonstrated less locomotor movements than in the beginning of the previous session. It is concluded that after short (30-min) exposures to a novel environment, crayfish can retained the information for at least four hours. Following a single 5-hour exposure, retention increases for at least 24 hours. Some similarities and differences in the behavior of the crayfish and rats in the “open field” are discussed.