Holococcoliths (calcareous nannoplankton constructed of identical 0.1-µm crystals) are reported for the first time from Mesozoic sedimentary rocks. Although most common and diverse during Maestrichtian and Campanian time, they appear to have been a common nannofloral component during much of the Cretaceous Period. Their earliest-known occurrence is in Hauterivian rocks, although they may range down to Upper Jurassic rocks. The most common Mesozoic forms, Lucianorhabdus and Phanulithus, represent the older portion of an evolutionary lineage that bridges the Cretaceous terminal event, being represented in Tertiary time by Zygrhablithus and related forms. The propensity for syndepositional dissolution of holococcoliths and their habit of serving as centers for secondary calcite overgrowth during diagenesis suggest that Mesozoic holococcoliths may serve as valuable indicators for paleobathymetry and for monitoring postdepositional diagenesis.