The skin reflectances of 257 Bolivian youths of European ancestry (140 males, 117 females; 9.0-19.9 yr) residing in Santa Cruz, Bolivia (average altitude of 400 m) are described. Reflectances were measured at three wavelengths (425 nm, 545 nm, and 685 nm) on two different sites, the medial surface of the inner upper arm and the forehead. Males tended to be darker than females, significantly on the inner upper arm (P < .05). Males tended to darken significantly with age on both the upper arm and the forehead (P < .05), while females tended to lighten with age on the upper arm and darken with age on the forehead. However, with the exception of reflectances at 545 nm on the inner upper arm, the age changes in females were insignificant (P > .05). The results of comparisons between two groups based on maternal and paternal surnames (two Spanish surnames, at least one non-Spanish European surnames) were consistent with expectations, or with those with two Spanish surnames tending to be significantly darker than those with one or more non-Spanish European surname (P < .05). Finally, the results of a principal components analysis of reflectances were similar to those found in a previous study of highland Bolivians of Aymara ancestry. Am. J. Hum. Biol. 10:559-565, 1998. ยฉ 1998 Wiley-Liss, Inc.