A 6-day in situ comparison between the Wetzel inorganic 14C uptake and Zieman leaf-biomass techniques for measuring net primary production rates in the seagrass Thalassia testudinum was performed in the northeastern Gulf of Mexico. Measurement differences between the two methods were insignificant when the 14C uptake technique was corrected for sediment 14C “uptake” (13%), incubation-chamber light-energy absorption (14%) and differences in total light-energy which resulted from the experimental design (7.7%). These results reinforce previous observations that the 14C technique estimates net particulate-carbon production.