The reactions of more than 200 zoo animals to a standardized set of novel objects were recorded and quantified. Our results indicated significant differences among various taxonomic groups, both in the quantity and form of object manipulation. Our major quantitative findings were as follows: A. Primates and Carnivores exhibited more investigatory behavior than Rodents or a group of "primitive" mammals. B. A sample of 20 reptiles showed very little response, with the exception of a single Orinoco crocodile. C. There were no significant overall sex differences among 53 mature pairs of mammals drawn from different orders. D. A sample of subadult mammals tended to be more reactive than adult members of the same species. E. Within the Old World monkeys, there was considerable variation between the two subfamilies: The Cercopithecinae indulged in more investigatory behavior than the Colobinae, while the latter exhibited a greater percentage of purely visual "orienting" responses. F. Among the Cercopithecinae, the baboons and macaques demonstrated more investigatory behavior than the guenons and patas monkeys, although the latter genera displayed a larger percentage of visual orienting responses. G. The limited sample of Prosimians did not differ significantly from the highly evolved Colobinae in total reactivity. H. There were no significant differences among the families of carnivores within our sample, although the smaller cats comprising the genus Felis were less reactive as a group than the larger cats of the genus Panthera. In addition, there was some hint that the meerkats, which constituted the only representatives of the Vivveridae, were less immediately reactive than the remaining carnivorous species. I. Among a relatively small sample of Rodents, the members of the suborder Hystricomorpha exhibited more reactivity than those in the suborder Myomorpha, with the Sciuromorpha in an intermediate position. In terms of the forms (motor sequences) of reaction, chewing constituted the primary contact response within all groups. However, the following additional reactions were noted: A. Primates were characterized by grasping with the forepaws, visual inspection, and manipulation of objects. The latter was particularly evident in the baboons and macaques where many "individualistic" patterns appeared. These, in turn, contrasted with the Prosimian group where such diverse manipulatory behavior was rarely, if ever, recorded. Indications of fear were observed in 19/100 adults. B. Carnivores showed a vigorous, relatively fearless, approach to the objects which approximated the patterns that might normally be used in the capture and consumption of prey: swatting with the paws, stalking, chasing, biting, "worrying," and tugging against the paws. C. Rodent reactions were generally limited to gnawing, although there appeared to be differences in the aggressiveness of approach among the three suborders. There were also some hoarding responses, (depositing objects in a particular part of the cage, or burying them in the sawdust). D. Some possibly species-characteristic techniques of object manipulation were noted, e.g., the use of the tongue and snout by the giant anteater, the tail by the spider monkey, and the lips by the guanaco. A preliminary attempt was made to interpret our results in relation to feeding patterns and danger from predators in the natural habitats of the species that we tested. The possible significance of species differences in reactivity for brain-behavior correlations, and contemporary behavioristic theory, was also briefly considered.