Deep-marine sediment has not been subjected to the influences of meteoric waters, deep burial, or tectonic stress that have profoundly altered most rocks available to us in continental exposures; hence, it preserves the record of formation and early development of its dolomite. Evaluation of the DSDP X-ray and smear slide data files indicates that dolomite is a common component in deep-marine sediments; it averages 1% and is found in all sampled ocean basins throughout post-Jurassic time. Dolomite abundance is not related to the abundance of any other detrital, authigenic, or alteration mineral, except aragonite. Petrographic examination of 48 samples suggests that deep-marine dolomite crystal size and appearance are similar to the size and appearance in modern supratidal deposits. Dolomite does not increase in abundance or in crystal size downhole, nor does crystal size increase with increasing age. The dolomite is commonly nonstoichiometric (avg. 56% CaCO 3 ) and poorly ordered. Globally, it does not change in stoichiometry or order with depth or age. Dolomite in deep-marine sediments does not originate from unusual or regionally important conditions. It entered the sediments early, most probably as a chemical precipitate from pore waters. An uncertain amount, estimated to be 10%, is detrital.