Pigment analyses have been made of a lagoon emptying into the Gulf of Mexico. This water showed a typical cycle of early spring flowering, later spring minimum, summer oscillations, and a secondary early fall maximum. The cycle resembled those of more northern waters, but the blooms in East Lagoon occurred earlier in the year. The bloom periods were related to various physical factors—solar radiation, temperature, and rainfall—with a direct relationship between incident radiation and chlorophyll a production indicated for the fall of 1957. The chlorophyll production of the lagoon compared favorably with other waters in these latitudes, yielding a mean of 17.6 mg chlorophyll a/m3during the sampling period from October 1957 to May 1959. An analysis of the precision of the method indicated a standard error of 0.565 ± 0.085 mg/m3for chlorophyll a and 1.35 ± 0.165 MSPU/m3for chlorophyll c. The large value for the latter pigment is attributed to the method of calculation.