Primary biological productivity in the coastal waters of Puerto Rico is based upon the synthetic processes of phytoplankton, benthic algae, mangroves, corals and the turtle grass, Thalassia testudinum. A considerable portion of the total production of organic matter in this region appears to be contributed by the large beds of Thalassia, widely scattered at depths of less than five meters, over well-illuminated, shallow bays, channels, and inner margins of coral reefs. These studies have been initiated in order to find out more about the magnitude, chemical composition, and possible value of Thalassia to the marine life of tropical regions. Some simple comparisons of proximate analyses are made between Thalassia, and other primary crops of grasses from Georgia, and phytoplankton from Long Island Sound. Standing crop. Determinations of the standing crop of Thalassia were made at representative stations near La Parguera during February of 1958. A special cutting instrument was used to obtain samples of plants including leaves, rhizomes and roots, from a uniform area of one square foot. Samples were washed in a sieve, dried in an oven at 1000C, and weighed. Typical results of the determinations are shown in Table 1. The range of