In order to determine the cognitive development of Navajo children in terms of Piagetian conservation of number, mass, and continuous quantity, 168 Navajo children at seven different age levels from 5 to adult were presented with a series of three conservation tasks. The tasks consisted of a standard object and an equivalent object that could be made to vary quantitatively and perceptually. The resulting incidence of conservation was low. Conservation was both age and task related. Subjects under 9 years of age did not conserve, but conservation increased towards adulthood, with adults measuring 78% conservers. More conservation occurred on the number task than on either of the others. The data support the Piagetian thesis that children develop cognitively according to a predetermined order of stages, but the low incidence of conservation among Navajo children suggests a "time-lag" in cognitive development for the group as compared to the dominant Western culture. This leads to the idea that Piagetian theory may be ethnocentric. (Author/SB)