Measurements of organic and inorganic carbon production were made for deep-water (76 m) specimens of Halimeda discoidea Descaisne, H. cryptica L. H. Colinvaux and Graham, H. copiosa Goreau and Graham and H. lacrimosa Howe. Photosynthetic and calcification rates (CaCO3 production) were estimated on shore at stimulated in situ light intensities (20 μE m−2 s−1) and temperatures (27°C). Photosynthetic rates, as measured by both oxygen evolution and carbon-14 incorporation, agreed well and showed efficient light utilization, ranging from 0.04 to 0.24 mg organic C g dry wt−1 h−1. Calcification rates, measured by 14C incorporation, ranged from 0.06 to 0.16 mg inorganic C g dry wt−1 h−1 and were positively correlated (R2=77) with photosynthesis, implying a physiological mediation of the depositional process. Of the four species, only H. cryptica exhibited significant (P<0.05) interspecific differences in calcification and photosynthetic rates (both rates being higher) due, in part, to its greater organic content. The carbon incorporation rates and associated algal cover suggest deep-growing Halimeda species to be important producers of carbonaceous materials.