A variety of common marine calcareous algae from the tropical and subtropical West Atlantic are shown to be subject to post-mortem disaggregation. The calcareous hard parts of many of these algae, including both green and red forms, consist wholly or partially of aragonite needles. These algally secreted needles are similar in habit and dimensions to sedimentary aragonite needles, all of which have previously been attributed to inorganic, physiochemical precipitation from sea water. In several widely scattered areas, the occurrences of such needlesecreting algae coincide with sediments that contain aragonite needles; this infers a largely algal origin for the sedimentary needles. These algae and associated sediments are not limited to the West Atlantic and probably occur through the tropical seas.