Carbon flux in arid and semiarid area shrublands, especially in old-growth shrub ecosystems, has been rarely studied using eddy covariance techniques. In this study, eddy covariance measurements over a 100-year old-growth chamise-dominated chaparral shrub ecosystem were conducted for 7 years from 1996 to 2003. A carbon sink, from −96 to −155 g C m−2 yr−1, was determined under normal weather conditions, while a weak sink of −18 g C m−2 yr−1 and a strong source of 207 g C m−2 yr−1 were observed as a consequence of a severe drought. The annual sink strength of carbon in the 7-year measurement period was −52 g C m−2 yr−1. The results from our study indicate that, in contrast to previous thought, the old-growth chaparral shrub ecosystem can be a significant sink of carbon under normal weather conditions and, therefore, be an important component of the global carbon budget.