Abstract Low aspartic acid d:l ratios and modern collagenlike concentration values indicate that amino acids in bones from the Rancho La Brea asphalt deposit, Los Angeles, California are better preserved than amino acids in bones of equivalent age that have not been preserved in asphalt. Amino acids were recovered from 10 Rancho La Brea bone samples which range in age from less than 200 to greater than 36,000 yr. The calibrated rates of aspartic acid racemization range from 2.1 to 5.0 × 10 −6 yr −1 . Although this wide range of rate constants decreases the level of confidence for age estimates, use of the larger rate constant of 5.0 × 10 −6 yr −1 provides minimum age estimates which fit the known stratigraphic and chronologic records of the Rancho La Brea deposits.