Abstract Seagrasses from various depths in St. Croix, U.S. Virgin Islands, and from shallow beds in Redfish Bay, Texas, were grown in the laboratory under three light conditions. For all collections, light absorption readings of extracted pigments showed that total chlorophyll content is inversely related to reduced light over the range from 35 to 200 μM m −2 s −1. The ratio of chlorophyll a to chlorophyll b decreases in response to reduced light for Caribbean collections of Halodule wrightii Aschers., Syringodium filiforme Kütz., and Halophila decipiens Ostenfeld but not for Thalassia testudinum Banks ex König or the Texas collections of Halodule, Syringodium, Thalassia, and Halophila engelmannii Aschers. There is a correlation of the maximum depth of the St. Croix seagrasses and the ratio of chlorophyll a to chlorophyll b: H. decipiens, with the greatest depth range, to −42 m, has the lowest ratio; T. testudinum, with the least depth range, to −12 m, has the highest ratio; H. wrightii and S. filiforme have intermediate depth ranges and rations. Although light quality and sea bottom characteristics may play roles in the ultimate depth to which a seagrass may occur, photon flux density is suggested as a primary environmental determinant.