Octopuses that have not been trained to come out to attack figures moving in the visual field do so only on some occasions. With repeated unrewarded trials at long intervals the frequency of attack does not increase even if the animals are left without food. There may be a slight decrease ('habituation'). The tendency to attack is not the same throughout the day, but is greater in the evening. The tendency to attack moving figures is greatly increased during the period of 1 to 2 h after an octopus has been fed, even if the food is given in the home and not at the place where the figures are shown. Other things being equal, there are marked differences in the tendency to attack various moving figures. Crabs were attacked more often than any other figures tested. Circles were attacked more than rectangles of the same area, white figures more than black and small rectangles more than large. There was some evidence of a tendency to attack vertical more than horizontal rectangles (both being moved vertically). The effect of removal of the vertical lobe is to change the previous tendency of the animals to attack. Of thirty animals operated, eighteen attacked more after operation, twelve attacked less. Those that attacked more afterwards had previously attacked on 26.5% of occasions, those that attacked less on 60.5% of occasions. The operation thus removes at least part of the memories set up before operation. Those animals in which these memories tended to promote attack will come out less often after operation and vice versa.