The effect of various environmental factors on net photosynthesis rates (P^) of the differentially depth distributed Mozambican seagrasses, Cymodocea serrulata (R.Br.) Aschers and Magnus (Zannich), and Thalassodendron ciliatum (Forsk.) den Hartog, was examined. At saturating irradiances the deeper water species, T. ciliatum, showed a maximum (as measured by short term O2 evolution) of 12.67 ± 3.33 mmol(02) kg-i(Chl) s ' with added 40 mM NaHCOj (0.89 mM dissolved CO2 at pH 8); the shallower species, C. serrulata, showed maximum rates of 21.67 ± 4.17 (10 mM NaHC03; 0.26 mM dissolved CO2). Saturating irradiances were similar for both species. PN estimated by the lacunal gas discharge technique was 16.00 ± 5.00 mmol(02) kg-'(Chl) s"’ for C. serrulata and 11.67 ±6.67 for T. ciliatum. Pn of both species was reduced by increased water depth, and the pattem of the effect of depth was similar.