We present data demonstrating constant δ13C values in 12 common planktonic foraminiferal species, which constitute >99% of the total specimens (>63 mu m) in assemblages collected across the Cretaceous-Tertiary (K-T) boundary at Caravaca, Spain. These latter values are in contrast to a rapid reduction in δ13C in samples of the fine fraction of carbonate at the K-T boundary and a subsequent negative shift of δ13C of a benthic foraminiferal species. These results indicate that the post–K-T occurrences of Cretaceous planktonic species in lower Danian sediments at Caravaca are the result of reworking. Rapid decreases in the percentage abundance of well-preserved specimens and in the number of specimens per gram of carbonate for the same 12 species at the K-T boundary also suggest reworking and abrupt extinction at the K-T boundary. Our data imply that sudden changes occurred within the pelagic ecosystem during the K-T boundary event.